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ELECTRICITY
Electricity is an important source of energy in he modern times. It is used in our homes, in industry, in transport etc.
There are two type of electric charges :positivecharge and negative charge. The charge acquired by a glass rod (rubbed
with a silk cloth) is called positive charge and the charge acquired by an ebonite rod (rubbed with a woolen cloth) is
called a negative charge.
The important property of electric charge is that :
i)
ii)
Opposite charge (or Unlike charge) attract each other. For e.g., a positive charge attract a negative charge.
Similar charge (or like charge) repel each other. For e.g., a positive charge repels a positive charge; and a
negative charge repels a negative charge.
The SI unit of electric charge ‘coulomb’ (C) is a equivalent to the charge contain in 6.25*10 18 electron.
Those substance through which electricity can flow are called conductor. Those substance through which electricity
cannot flow are called insulators.
Electric potential
The electric potential at a point in an electric field is defined as the work done in moving a unit positive charge from
infinity to that point. potential is denoted by the symbol V and its unit is volt.
A potential or electric potential of 1 volt at a point means that 1 joule of work is done in moving 1 coulomb of positive
charge from infinity to that point.
Potential difference
The difference in electric potential between two points is known as potential difference. The potential difference
between two points in an electric circuit is defined as the amount of work done in moving unit charge from one point
to the other point. That is :
𝑤𝑜𝑟𝑘 𝑑𝑜𝑛𝑒
Potential difference= 𝑄𝑢𝑎𝑛𝑡𝑖𝑡𝑦 𝑜𝑓 𝑐ℎ𝑎𝑟𝑔𝑒 𝑚𝑜𝑣𝑒𝑑
If W joules of work has to be done to move Q coulomb of charge from one point to the other point, then the potential
difference V between the two points is given by the formula :
Potential difference,
V=
𝑊
𝑄
Where, W = work done
And Q= quantity of charge moved
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The SI unit of potential difference is volt (V). the potential difference between two point is said to be 1 volt if 1 joule
of work is done in moving 1 coulomb of electric charge from one point to the other.
Thus, 1 volt =
𝟏 𝒋𝒐𝒖𝒍𝒆
1𝐽
Or, 1 v = 1 𝐶
𝟏 𝒄𝒐𝒖𝒍𝒐𝒎𝒃
Or, 1v = 1J C-1
The potential difference is measured by means of an instrument called voltmeter.
Electric current
The rate of flow of electric charge is called electric current. The magnitude of electric current in a conductor is the
amount of electric charge passing through a given point of the conductor in one second.
𝑄
Current, I =
𝑇
The SI unit of electric current is Ampere which is denoted by the letter A. When 1 coulomb of charge flow through any
cross-section of conductor in 1 second, the electric current flowing through it is said to be 1 ampere. That is,
1 ampere =
1 𝐶𝑜𝑢𝑙𝑜𝑚𝑏
Or1
1 𝑆𝑒𝑐𝑜𝑛𝑑
1𝐶
A = 1𝑆
A smaller unit of current called “milliampere” denoted by ‘mA’
1 ‘ma’= 10-3 A
current is measured by an instrument called Ammeter.
Ohm’s law
At constant temperature, the current flowing through a conductor is directly proportional to the potential difference
across its ends. If I is the current flowing through a conductor and V is the potential difference across its ends, then
according to Ohm’s law :
I directly proportional V
Or
V = R*I
It is obvious from this relation that :
i)
ii)
The current is directly proportional to potential difference,
The current is inversely proportional to resistance.
Resistance of a conductor :
the property of a conductor due to which it opposes the flow of current through it is called resistance.
Resistance =
𝑝𝑜𝑡𝑒𝑛𝑡𝑖𝑎𝑙 𝑑𝑖𝑓𝑓𝑒𝑟𝑒𝑛𝑐𝑒
𝑐𝑢𝑟𝑟𝑒𝑛𝑡
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The resistance of conductor depends on length, thickness, nature of material and temperature , of a conductor.
The SI unit of resistance is ohm which denoted by Ω, omega. 1 ohm is the resistance of a conductor such that when a
potential difference of 1 volt is applied to its ends, a current of 1 ampere flows through it. That is :
1 ohm =
1 𝑣𝑜𝑙𝑡
1 𝐴𝑚𝑝𝑒𝑟𝑒
Factor affecting the resistance of a conductor
The electrical resistance of a conductor depends on the following factors :
i)
ii)
iii)
iv)
Length of the conductor.
Area of cross-section of the conductor
Nature of the material of the conductor
Temperature of the conductor
Resistivity
A measure of resisting power of a specified material to the flow of an electric current.
R= p
𝒍
𝑨
Or
Resistivity, p =
𝑅∗𝐴
𝑙
The SI unit of resistivity is ohm-meter which is written as Ωm.
Note: the resistivity of a substance does not depend on its length or thickness. It depends on the nature of the substance
and temperature.
Resistance in series :
The combined resistance of any number of resistance connected in series is equal to the sum
of the individual resistance. So the formula is:
RS = R1 + R2 + R3 +…..
Resistance in parallel :The reciprocal of the combined resistance of a number of resistances connected in parallel is
equal to the su of the reciprocals of all the individual resistances.so the formula is :
𝟏
𝑹
𝟏
𝟏
𝟏
= 𝑹 + 𝑹 + 𝑹 +……….
𝟏
𝟐
𝟑
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Disadvantage of series circuit :
1) In the series circuit, if one electrical appliance stops working due to some defect, then all other appliances also
stops working.
2) In series circuit, all the electrical appliances have only one switch due to which they cannot be turned on or off
separately.
3) In series circuit, the appliances do not get the same voltage as that of the power supply line.
4) In the series connection of electrical appliance, the overall resistance of the circuit increases too much due to
which the current from the power supply is low.
Effect Produced By Electric Current :An electric current can produced three important effects. These are :
(i)
(ii)
(iii)
Heating effect
Magnetic effect
Chemical effect
Now we have to study the heating effect.
Heating Effect Of Electric Current
When an electric current is passed through a high resistance wire, the resistance wire become very hot and produces
heat. This is called the heating effect of current. The heating effect of current is obtained by the transformation of
electrical energy into heat energy.
H= I2 × R × t joules
This formula gives us the heat produced in joules when a current of I ampere flows in a wire of resistance R ohms for
time t seconds. This is known as joule’s law of heating. According to joule’s law f heating given by formula H = I2 × R × t,
it is clear that the heat produced in a wire is directly proportional to :
i)
ii)
iii)
Square of current
Resistance of wire
Time, for which current is passed
Note :all the appliances which run on electricity do not convert all the electric energy into heat energy.
Application of the heating effect of current :

The heating effect of current is utilized in the working of electrical heating appliances such as electric iron,
electric kettle, toaster, etc.


The heating effect of electric current is utilized in electric bulbs for producing light.
The heating effect of electric current is utilized in electric fuse for protecting household wiring and electrical
appliances.
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What is a Magnet ? “MAGNETISM”
A piece of iron or other material which has its component atoms so ordered that the material exhibits properties of
magnetism, such as attracting other iron-containing objects or aligning itself in an external magnetic field.
Bar Magnet - A bar Magnet is a rectangular object that has a magnetic field.
Magnetic field is certain region of space around a magnet with in which the magnetic effects and attractions of the
objects and elements can be felt. Or Region around a magnet where the magnet exerts its influence.
Properties of bar magnet
– It align to itself in the North-South direction when suspended freely. – North pole and south pole cannot be isolated.
– Likes poles repel and unlike poles attract each others.
– Not all things are attracted by magnets. (Magnets Attracts – like Iron, nickel, cobalt and neodynium etc)
A magnetic field is the magnetic effect of magnetic materials, and the magnetic effect is represented by lines that are
known as magnetic field lines.
Or A Magnetic field is the space surrounding a magnet in which magnetic force is exerted.
Magnetic field lines are the lines drawn in a magnetic field along which a north magnetic pole would move.
Properties of magnetic field lines- :They form continuous closed loops.
: Tangent to the magnetic field line at a given point specifies the direction of the net magnetic field at that point.
: Greater the number of magnetic field lines per unit area, stronger is the magnetic field.
:Magnetic field lines never intersect each other(suppose two magnetic field lines intersect each other but when we
draw their two tangents then we have two tangents for a same common point that means we have 2 direction for
magnetic field that is not possible).
Oersted Experiment (1820) – In this experiment scientist name Oersted tried to establish a relationship between
electricity and magnetism. Before this experiment there was no relation between electricity and magnetism.
Observation – :When current is passed through a wire the direction of the magnetic needle of compass showed
deflection.
:As the amount of current varied it increased the deflection increased and vice a versa.
:In this experiment it was proved that magnetic field is produced around conductor when current is passing through it.
:Deflection reverses as the electric current is reversed.
Conclusion – A wire carrying electric current behaves like a magnet. :Moving charges produce magnetic field in the
surrounding region.
Magnetic Field [M.F.] Produced due to a current carrying conductor - Straight wire,
, Circular Loop Solenoid produces magnetic field when current is passed through them .
M.F through a straight current carrying conductor. –
:Magnitude of the magnetic field produced at a given point increases as the current through the wire increases.
:Magnetic field produced by a given current in the conductor decreases as the distance of the point from it increases.
:Pattern of the magnetic field line would be cocentric circles around the current carrying straight wire.
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:Direction of Magnetic field in straight current carrying conductor is checked by Right Hand Thumb
Rule [Thumb of right hand tell the direction of current and the fingers tell direction of M.F].
(Straight Conductor)
M.F through Current carrying circular loop conductor – :Every point on the wire carrying current
would give rise to magnetic field appearing as straight lines at center of loop.
:The far we will go from the current carrying conductor the curvature of the
M.F. lines reduces and at farthest point they become straight. And in between
the circular loop also it forms a straight line. Any point on circular loop will
behave in same manner.
:Net magnetic field would be perpendicular to plain of the wire at the center of
wire.
Direction of M.F in circular loop - Would be checked By Right Hand Screw rule.
:At the centre of the circular loop, all the magnetic field lines are in the same
direction and aid each other , due to which the strength of magnetic field
increases.
The Magnitude of magnetic field produced by current carrying circular loop is – i)directly proportional to the current
passing through the circular loop or wire , ii)inversely proportional to the radius of the circular wire.
:Direction of the M.F. would be perpendicular to the circular loops and Upwards.
M.F due to a solenoid- It is a long coil containing a large number of close turns of insulated copper wire. It is also called a
coil wound into a tightly packed helix. We used solenoids as electromagnets, and we use them in transformer creation.
The magnetic field produced by a current carrying solenoid is similar to the magnetic field produced by a bar magnet.
The magnetic field inside a solenoid is in the form of straight line, and that field is uniform inside solenoid.
:Strong magnetic field inside solenoid is used to magnetize magnetic materials like, soft iron, when placed inside the coil.
Important features of current carrying solenoid1.The magnetic field lines inside the solenoid are nearly straight and parallel to its axis. It shows that magnetic field
inside a solenoid is uniform.
2.The magnetic field of solenoid is identical to bar magnet with one end of solenoid behaving as a north pole and other
end as a south pole. It shows that a solenoid behaves like a bar magnet.
3.Acurrent carrying solenoid exhibits the directive and attractive properties of a
bar magnet.
MAGNETIC FORCE– When a current carrying wire is placed in a magnetic
field, a mechanical force is exerted on the conductor which makes it move and
that force is called Magnetic force.
Or Force experienced by a current carrying wire when placed in a magnetic
field or the force experienced by a charged particle moving in a magnetic field
is known as the Magnetic force.
The direction of the force acting on a current carrying wire placed in a magnetic field is (i)Perpendicular to the direction
of current. (ii)perpendicular to the direction of the magnetic field.
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: The maximum force is exerted on a current-carrying conductor only when it is perpendicular to the direction of
magnetic field. No force acts on a current carrying conductor when it is parallel to the magnetic field.
: The direction of force on a current carrying conductor placed in a magnetic field can be reversed by reversing the
direction of current flowing in the conductor.
: The direction of force on a conductor can also be changed by reversing the direction of the magnetic field.
F.L.H (Fleming’s Left Hand Rule )
Adjust your left hand in such a way that the forefinger points in the direction of magnetic field and the center finger
points in the direction of current, then the direction in which thumb points. Gives the direction of force acting of the
conductor and all the finger adjusted should be adjusted and pointed by making angle of 90 degree .
Q1 A stream of positively charged particles moving towards west is deflected towards north by a magnetic field. The
direction of magnetic field would be ?
A)Positive particles are moving in west, so the direction of current is towards west, the deflection is towards north, so
the force is towards north, the direction of magnetic field will be upwards.
F.R.H (Fleming’s Right Hand Rule) for direction of induced currentHold the thumb, the forefinger and the center finger of right hand at right angles to one another. Adjust your hand in
such a way that forefinger points in the direction of magnetic field, and thumb points in the direction of motion of
conductor and, then the direction in which centre finger points gives the direction of induced current.
:The Production of electricity from magnetism is called E.M.I. (electro magnetic induction)
:The production of electric current by moving a magnetic field inside a fixed coil of wire is also a case of E.M.I
:The condition necessary for the production of electric current by E.M.I is that there must be a relative motion between
the coil of wire and a magnet.
Faraday and Henry Experiment’s observations
The observation made were as follows –
1.A current is induced in a coil when it is moved or rotated relative to a fixed magnet.
2.A current is also induced in a fixed coil when a magnet is moved or rotated relative to the fixed coil.
3.No current is inducted in a coil when the coil and magnet both are stationary relative to one other.
4.When the direction of motion of coil or magnet is reversed, the direction of current induced in coil also get reverse.
5.The magnitude induced in coil can be increased :
a.)by winding the coil on a soft iron core.
b.)by increasing the number of turns of the coil.
c.)by increasing the strength of the magnet.
d.)by increasing the speed of rotation of coil.
A Galvanometer is a instrument which can detect the presence of electric current in a circuit.
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D.C and A.C. current (Direct Current and Alternating Current)
If the current flows in one direction only, it is called Direct Current. Ex- Battery and Cell ,in this positive and negative
terminals are fixed. The magnitude of current may or may not remain constant.
When current reverses its directions after equal intervals of time , it is called Alternating Current. The alternating current
produced in India reverses its direction in every 1/100 seconds. So that’s why positive and negative terminal of A.C is not
fixed. Magnitude of current continuously changes with time.
Advantage of A.C. over D.C is that Alternating current can be transmitted over long distances without much loss electric
energy.
ELECTRIC GENERATOR– It converts mechanical energy into electrical energy.
Are of two types : A.C generator – Alternating current is produced which changes direction after equal intervals of time
and D.C. generator- Direct current is produced which does not changes its direction with time.
Principle – It works on the principle that when a straight conductor is moved in a magnetic field, then current is induced
in the conductor. Current is induced in a loop through a change in its orientation or a change in its effective area.
AC GENERATORConstruction- Rectangular coil of insulated copper, Magnets, Split ring- Split ring commutator is used to ensure
unidirectional flow of current, Axle, brushes and Galvanometer (to detect presence of current generated).
Fleming’s Right Hand Rule is used in Electric Generators.
ELECTRIC MOTOR– It converts electrical energy to mechanical energy.
Principle – Motor works on the principle that when a rectangular coil is placed in a magnetic field and current is passed
through it, a force acts on the coil which rotates it continuously.
Construction - :Rectangular coil of insulated copper . :Magnets. :Split Rings. :Axle. :Brushes. :Source Battery
In commercial motors- Electromagnets, More turns of conducting wire on coil, Soft iron core on which coil is wound.
Significance of split rings- :They act as commutator, Commutator is a device that reverses current, Reversal of current
results in continuous rotation of the coil.
Fleming’s Left hand Rule is used in Electric Motor.
Advantages of parallel Wiring in Houses.
1.)The main advantage is that if one appliance is switched off or it fuses , then there is no effect on other appliances and
they keep on operating.
2.)Same voltage of the mains line is available for all the electric appliances.
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Q)Show experimentally that change in current flowing through a coil causes electromagnetic induction in a neighboring
coil ?
ans)Take 2 different coils of insulated copper wire having large number of turns(50 or more). Insert the two coils over a
non-conducting cylindrical thick paper roll.
Connect 6 volt battery and a plug key in series of coil 1, and in coil 2 connect a galvanometer. Now switch on the key ,
the galvanometer with coil-2 also gives a momentary deflection and then the pointer quickly returns to its mean
position.
Now remove the plug form key , the galvanometer in coil 2 again gives a momentary deflection but in reverse direction.
Moreover we observe that as soon as current in coil 1 becomes either steady or zero, there is no deflection in
galvanometer. From the above experiment, we conclude that an induced current is produced in coil 2 on account of
electromagnetic induction whenever current in coil 1 is changing.
Q) How will you explain the phenomena of electromagnetic induction in one coil when current passing through a
neighboring coil increase or decreases ? What is mutual inductance ?
Ans) (draw the above figure in this question also)
We know that current is induced in a coil whenever current flowing through a neighboring coil changes. As shown in
figure let current in coil 1 changes, due to change in current the magnetic field produced around coil 1 also changes
accordingly. As coil 2 is situated very close, magnetic field around it also changes. Due to change in magnetic field around
it, an induced current is setup in
coil 2.
Generally coil 1 is called primary
coil and coil 2 is called secondary
coil, this phenomenon, be which
a changing magnetic field in a
conductor induces a current in
another conductor, is know as
“mutual inductance”.
Q A student fixes a sheet of white paper on a drawing board, he places a bar magnet in the center of it, he sprinkles
some iron-fillings uniformly around the bar magnet, then he taps the board gently.
Now answer the following questions :(a)What does the student observe (b)why do the iron filling arrange is such a pattern (c) why does the crowding of the
iron filling at the ends of magnets indicate ?
Ans) (a) Student observes that iron filling arrange in a definite pattern around the magnet as shown in figure.
(b) the magnet exerts its influence in the region surrounding it, therefore the iron fillings experience a force, due to this
force the iron fillings get arranges is such a pattern, the line along which the iron fillings align themselves represent
magnetic field lines around the bar magnet.
(c) the crowding of the iron fillings at the end of the magnet indicates that magnetic field there is the strongest.
Q)Name some common devices that use current carrying conductors and magnetic fields?
Ans) Some common devices that use current carrying conductors and magnetic fields are electric motor, electric
generator, loud speakers, microphones and measuring instruments like galvanometer, ammeter, voltmeter etc.
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Q)What is the usual colour code followed for connecting live,
neutral and earth wires? Why it is important ?
Ans) As per usual colour code followed the live wire is of red
coloured insulation, neutral wire of black coloured insulation
and earth wire of green coloured insulation. If this colour
code is followed then any person would identify the correct
connecting wire and would be able to handle them properly.
Q)What is role of fuse ? why should a fuse with defined rating
not be replaced by one with a larger rating ?
Ans) The fuse is a safety device used to prevent any damage to an electrical appliance either due to short circuiting or
overloading of the electric current. If a fuse, with defined rating, is replaced by one with a larger rating then the fuse
wire will not burn even when a current greater than safe limit is flowing. As a result the electrical appliances will be
damaged.
Q)An alternating current has a frequency of 50Hz. How many times does it change its direction in one second?
Ans) 50 X 2 = 100 times. (as time of on A.C. cycle is 2)
Q)what is the important characteristics of a fuse wire ?
Ans) Melting point of the material of fuse wire should be extremely low.
Q)What will be the frequency of an alternating current, it its direction changes after every 0.01 seconds ?
Q) Describe an experiment in detail to demonstrate the force acting on a current carrying conductor due to a magnetic
field. Also show the effect of change in direction of magnetic field and change in direction of current ?
Assignment PART 1
Q1)In which direction does a freely suspended compass needle align itself?
Q2)What is the direction of magnetic field at a given point ?
Q3)How can it be shown that a magnetic field exists around a wire through which a electric current is passing ?
Q4)Where will be the value of magnetic field maximum due to current carrying circular conductor ?
Q5)Which current carrying device behaves like a bar magnet and is has same Magnetic field lines as of bar magnet ?
Draw neat diagram to show it ?
Q6)Define induced current ?
Q7)Under what conditions is the force be a current carrying conductor placed in a magnetic field is maximum ?
Q8)Why do we connect earth wire in houses ? give 2 reasons ?
Q9)What modification is done in AC generator so as to produce DC current ?
Q10)Name a metal which offers higher resistance to the passage of electricity than copper ?
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PART 2
Q. 1. What is a magnetic field?
Q. 2. What are the magnetic lines of force?
Q. 3. What is an electric motor?
Q. 4. What is a solenoid?
Q. 5. Which effect of electric current is utilized in the working of an electric motor?
Q. 6. What is the frequency for a.c (alternating current) in India?
Q. 7. On what principle is an a.c generator based?
Q. 8. Why don’t two magnetic lines of force interest each other?
Q. 9. Name some source of direct current?
Q. 10. When does an electric short circuit occur?
Q. 11. What is the usual colour code followed for connecting positive, neutral and earth wires. Why is it so
important?
Q. 12. What are electromagnetic inductions?
Q. 13. State the rule to determine the direction of magnetic field produced around a current carrying
conductor?
Q. 14. What is the role of a fuse in the electric circuits?
Q. 15. Explain direct and alternating current?
Q. 16. What is the function of an earth wire? Why is it necessary to earth metallic appliances?
Q. 17. Draw magnetic field lines around a bar magnet?
Q. 18. Why does a compass needle get deflected brought near a bar magnet?
Q. 19. List the properties of magnetic lines of force?
Q. 20. What is the principle of an electric motor?
Q. 21. State the principle of an electric generator.
Q. 22. Two circular coils a and b are placed closed to each other. If the current in coil a is changed, will some
current be induced in the coil b? Give reason.
PART 3
Q.1 The magnetic field inside a long straight solenoid carrying current: (a) is zero (b) decreases as we
move towards its end (c) is same at all points. (d) Increases as we move towards its end
Q.2 Which of the following properties of proton can change while it moves freely in a magnetic field?
(a) mass (b) speed (c) velocity (d) momentum.
Q.4 A positively-charged particle (alpha particle) projected towards west is deflected towards north by
magnetic field. The direction of magnetic field is :
(a) towards south (b) towards east (c) downward (d) upward
Q.5 What is the role of the split-ring in an electric motor?
Q.6 What will be the frequency of an A.C if its direction changes after every .01 s?
Q.7 An A.C has a frequency of 50 Hz. How many times does it change its direction in one second?
Q.8 A student performs an experiment to study the magnetic effect of current around a current
carrying straight conductor. He reports that
(i) The direction of deflection of the north pole of a compass needle kept at a given point near the
conductor remains unaffected even when the terminals of the battery sending current in the wire are
inter changed.
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(ii) for a given battery, the degree of deflection of a N-pole decreases when the compass is kept at a
point farther away from the conductor. Which of the above observations of the student is incorrect
and why?
Q.9 Draw the pattern of magnetic field lines of a current carrying solenoid. What does the pattern of
field lines inside the solenoid indicate? Write one application of magnetic field of current carrying
solenoid.
Q.10 Sketch magnetic field lines around a current carrying straight conductor.
Q.11 Why does a current carrying conductor kept in a magnetic field experience force? On what
factors does the direction of this force depend? Name and state the rule used for determination of
direction of this force.
Q.12 Two circular coils A and B are placed close to each other. If the current in the coil A is changed,
will some current be induced in the coil B? Give reason.
Q13 Explain what is short-circuiting and overloading in an electric supply?
Q.14 What is the function of an earth wire? Why is it necessary to earth the metallic appliances?
Q.15 (a) What is an electromagnet? What does it consist of?
(b) Name one material in each case which is used to make a (i) permanent magnet (ii) Temporary
magnet.
Q.16 Draw a sketch of the pattern of field lines due to a (i) current flowing in a circular coil (ii) current
carrying solenoid.
Q.17 A circuit has a fuse of 5A. What is the maximum number of 100W, 220V bulbs that can be safely
used in the circuit.
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LIGHT – REFLECTION OF LIGHT
Light travels in a straight line and can change its direction when incident on a shiny surface.
Or
Light is a form of energy, light is needed to see things around us. (we detect light from our eyes.)
-Light travels along a straight path.
-Ex of sources of light – Sun, Bulb , CFL, Tube light, Torch, Lantern, Candle, LED, Fire and Stars etc.
Ray of Light – Straight line of path is termed as ray of light.
Beam Of light – Bunch of rays of light are known as beam of light.
Reflection Of light – Bouncing back of ray of light when it strikes a boundary between different media through
which it cannot pass.
-Light is needed for reflection of light.
Ex- Mirror, lake and rivers etc (are all examples of reflecting surface)
( Fig-2 )
Reflection of light follows 2 laws :
1)First law of reflection – The incident ray, the reflected ray and the
normal , all lie on the same plane.
2)Second law of reflection – The angle of incidence is always equal
the angle of reflection.
Means Angle of incident ray = angle of reflection . ( Θ1 = θ2)
to
( Fig-1 )
A spherical mirror can be made from a spherical ball.
-Spherical mirror is a part of a reflective surface. There are 2 types of mirrors
i) Concave mirror – It is also known as converging mirror. It is curved inwards and the reflection takes place in
from the inner surface. It resembles shape of a cave.
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ii) Convex mirror – It is also known as diverging mirror. Its reflecting surface is outwards, and reflection of light
takes place from its outer surface.
Figure -2. Showing How Spherical mirrors are part of a Hollow Sphere.
Terminologies –
Pole – Centre of the reflecting surface. Denoted by “P”
Centre of Curvature – Centre of sphere which the mirror is
partDenoted by “C”.
Principal Focus – Point on principal axis where the rays of light
incident on the mirror parallel to the principal axis converge or
appear to converge.Denoted by “F”
Principal axis – Line joining the pole with the centre of the
curvature is known as principal axis.
Radius of Curvature – Radius of the sphere of which mirror is a
part.
The focus of a spherical mirror always lies between the pole (P)
and the centre of curvature (C).
Relationship between radius of curvature and focal length of spherical mirror. – Focal length of a spherical
𝑅
mirror (of a concave mirror or convex mirror) is equal to the half of its radius of curvature. “ f = “.
2
Where R = radius of curvature, f = focal length.
Rules which are followed incase of mirrors for
image formation
Rule 1) Whenever a ray of light is incident on the
mirror parallel to the principal axis, it passes
through the principal focus.
Rule 2) A ray of light which passes through the
principal focus towards the spherical mirror
afterreflection is reflected parallel to the
principal axis.
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Rule 3) A ray of light which passes through centre
of curvature of a spherical mirror after reflection
retraces the same path, that is , goes back along
the same path.
Rule 4)A ray of light which is incident towards
the pole of the spherical mirror obliquely to
the principal axis is reflected obliquely.
Image formation by mirrors –
1)Image formation by Concave Mirrors. - A concave mirror can produce both real and virtual images. The
nature of the image depends primarily on the distance of the object from the mirror.
I. When the object is at infinity.II. When the object is behind the centre of curvature.
III. When the object is at the centre of curvature.
IV. When the object is between the centre of curvature (C) and focus (F).V. When the object is at focus (F).
VI. When the object is placed between the focus (F) and the pole (P).
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Table for Image formation by Concave mirrors –
2)Image formation by Convex Mirrors - A convex mirror always produces virtual and erect images of very small
size. The images formed by a convex mirror are primarily classified into two ways.
a). When the object is at infinity.- In this case, the image is appearing to form at focus. This image is virtual,
erect, and very small in size.
b)When the object is between pole (P) and a point X (X lies beyond C).- In this case, the image is formed
between the pole (P) and focus (F), behind the mirror. This image is virtual, erect, and small in size.
Sign Conventions for spherical Mirrors:
I.Objects are always placed to the left of the mirror i.e. light must fall on the mirror from left to right.
II. All distances are measured from the pole of the mirror.
III.Distances along the direction of the incident ray (along positive x- axis) are taken as positive, while distances
along the direction of the reflected ray (along negative x-axis) are taken as negative.
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IV.Heights measured perpendicular to and above the principal axis (along positive y-axis) are taken as positive.
V. Heights measured perpendicular to and below the principal axis (along negative y-axis) are taken as
negative.
Mirror formula – Relates image distance with object distance and focal length.
The distance of an object from the pole of a mirror is termed as the object distance
denoted by ‘u’.
The distance of an image from the pole of a mirror is termed as the image distance
denoted by ‘v’.
The formula relating the object distance (u), image distance (v), and the focal length (f) of a spherical mirror is
The following table summarizessign conventions for concave and convex mirrors.
Magnification of spherical mirror –
Magnification is a measure of how large or how small an image is with respect to the object.
Magnification is expressed by the ratio of the image height (Hi) to the object height (Ho).
𝐻𝑖 𝐻𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑡ℎ𝑒 𝑖𝑚𝑎𝑔𝑒
Magnification =
=
(also known as Linear Magnification)
𝐻𝑜 𝐻𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑡ℎ𝑒 𝑜𝑏𝑗𝑒𝑐𝑡
−𝑣 𝐼𝑚𝑎𝑔𝑒 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒
orMagnification = =
𝑢
𝑜𝑏𝑗𝑒𝑐𝑡 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒
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If m = +ve (then image is virtual and erect)
m= -ve (then image is real and inverted)
Assignment –
Q1. For the same angle of incidence 45 degree, the angle of refraction in two transparent media 1 and 2 is 20 degree and
30 degree respectively. out of 1and 2 which is optically denser medium and why ?
Q2. For the same angle of incidence in media p , q , r the angles of refraction are 45 , 35 and 15 degrees respectively. In
which medium will the velocity of light be minimum?
Q3.For what angle of incidence, the lateral shift produced by parallel sided glass plate is zero?
Q4. A coin in a glass beaker appears to rise as the beaker is slowly filled with water. Why?
Q5. An object under water appears to be at lesser depth than in reality. Explain why?
Q6.Why does bending of light takes place ?
Q7.Does the refractive index of substance change with the colour of light ?
Q8.What is the advantage of "total internal reflection" over reflection ?
Q9.In refraction of light through a rectangular glass slab, emergent ray is parallel to direction of the incident ray. why ?
Q10.Under what condition in an arrangement of two plane mirrors, incident ray and reflected ray will always be parallel
to each other, whatever may be angle of incidence. Show the same with the help of diagram.
Q11.Why does a driver prefer to use a convex mirror as a rear view mirror in a vehicle?
Q12.An object of 5cm height is placed at a distance of 10cm from a convex mirror of radius of curvature 30cm. find
nature , position and size of the image ?
Q13.An object of 20 cm from a spherical mirror gives rise to a virtual image 15cm behind the mirror. Determine the
magnification of the image and the type of mirror used.
Q14)pg – 168 all unsolved NCERT questions.
Q15)pg-171 all unsolved NCERT questions.
Q17)Write all the laws of reflection along with diagrams.
Q18)Draw all the ray diagrams for image formation by concave and convex mirrors.
Q19)What do you mean by magnification explain ? along with the formula ?
Q20)Describe all the rules for image formation in case of mirrors along with the diagrams ?
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LIGHT – REFRACTION OF LIGHT
Bending of light when it passes from one medium to another medium is known as refraction of light.
Or
When a ray of light travels from one transparent medium to another, it bends at the surface, thereby separating the two
media. Hence, the lemon appears larger than its actual size. This happens because different media have different optical
densities. The phenomenon of bending of light as it travels from one medium to another is known as refraction of light.
-The phenomenon of refraction shows that the speed of light is different in different media.
Refractive index is unique for every medium, every medium has a value of its own.
-By refractive index we judge the Bending of light.
-It is the parameter by which we get to know optical density of a medium.
-Refractive index is the index which defines the rate of refraction.
-Refractive index is the ratio of velocity of light of a given wavelength in empty space to its velocity in a medium.
-When ray of light goes from one medium to other medium then the Refractive index is called Relative Refractive index.
-When light is going from vacuum to other medium then value of Refractive index is called Absolute Refractive index.
-Refractive index of a substance or medium with respect to air is also considered as its Absolute Refractive index.
As a ray of light moves from an optically rarer medium to an
optically denser medium, it bends towards the normal at the
point of incidence. Therefore, the angle of incidence (i) is
greater than the angle of refraction (r). Hence, i > r
As a ray of light moves from an optically denser
medium to an optically rarer medium, it bends away
from the normal. Therefore, the angle of incidence (i)
is less than the angle of refraction (r).Hence, i < r.
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Laws of refraction - There are two laws of refraction.
First law of refraction- The
ratio of the sine of the angle
of incidence to the sine of the
angle of refraction is
constant. This is known as
Snell’s law. Mathematically.
Second law of refraction - The incident ray, the refracted ray, and the normal to the interface of two media at the point
of incidence – all lie in the same plane.
Grasshopper – frog relation
Consider a situation where a frog is sitting inside a pond (refractive indexµ2 )
while a grasshopper is sitting on a bush slightly above in air (refractive index μ1),
as shown in the given figure.
Situation I-The grasshopper is looking towards the frog.
In this case, light is travelling from air to water. The refractive index of air (μ1)
is less than that of water (μ2). Therefore, light gets slightly bent towards the
normal. Hence, the frog appears closer to the grasshopper.
Situation II - The frog is looking towards the grasshopper. In this case, light
is travelling from a denser to a rarer medium as refractive index of water
(μ2) is greater than that of air (μ1). Therefore, light will bend away from the
normal. Hence, the
grasshopper appears
farther to the frog.
Refraction through a glass slab
-When a light ray enters from air (rarer medium) to glass (denser
medium), it bends towards the normal.
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-When a light ray emerges from the glass (denser medium) to air (rarer medium), it bends away from the normal.
Hence, when a light ray is incident on a rectangular glass slab, the light emerges parallel to the incident ray from the
opposite side of the slab. And when a light ray is incident on a glass slab normally, it gets out straight without any
deflection i.e., i = 0, r = 0.
(Lateral Shift/Lateral Displacement is the phenomena in which the emergent ray shift laterally from the actual incident
ray due to the phenomena of refraction)
Total Internal Reflection(TIR)- It is the complete reflection of light ray at the boundary of two media when the ray
isin the medium with greater refractive index. OR
When light travels from a denser medium to a rarer medium and is incident at an angle more than the critical angle
for that medium, it is completely returned inwardly in the denser medium. This complete inward return of light is
called TIR.
Emergent ray is parallel to incident raybecause the extent of bending of ray of light at the opposite parallel side
of rectangular glass is equal and opposite.
Refraction Of Light By Spherical Lenses (Concave And Convex)
A lens is a transparent material bound by two curved
surfaces. Lenses are broadly classified into two
categories depending on their surfaces.
Convex lens - A convex lens is made by joining two
spherical surfaces in such a way that it is thicker at the
centre. Its thickness gradually reduces as we move
towards the edge.
A convex lens has the ability to converge the light rays
to a point that are incident on it. Thus, it is called a converging lens.
Concave lens - A concave lens is made by joining two curved surfaces in such a way that it is thinner at the centre. Its
thickness gradually increases as we move towards the edge.
A concave lens has the ability to diverge a beam of light rays incident on it. Thus, it is called a diverging lens.
Differences between a spherical mirror and a lens
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Terminologies Associated with Lenses
Optical centre - Optical centre is a point at the centre of the lens. It always lies inside the lens and not on the surface. It
is denoted by ‘O’.
Centre of curvature - It is the centre point of arcs of the two spheres from which the given spherical lens (concave or
convex) is made. Since a lens constitutes two spherical surfaces, it has two centre of curvature. The distance of the
optical centre from either of the centre of curvatures is termed as the radius of curvature.
Principal axis -The straight line joining the two centre of curvature and the optical centre (O) is called the principal axis
of the lens.
Focus -The focus (F) is the point on the principal axis where all incident parallel rays meet. The distance between the
focus (F1 or F2) and the optical centre (O) is known as the focal length of the lens.
Rules for obtaining images from Refraction by Spherical Lenses
Case I. When the incident
light ray is parallel to the
principal axis In this case,
the refracted ray will pass
through second focus F2 for
a convex lens, and appear to
diverge from the first focus
F1for concave lens.
Case II. When the incident light ray
emerges from the first focus F1 of a
convex lens, or appears to emerge
from the second focus F2 of a
concave lens. In this case, light after
refraction from both the lenses will
move parallel to the principal axis.
Case III. When the light ray passes
through the optical centre (O) of a lens In
this case, the light ray will pass through
both the lenses without suffering any
deviation.
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Image Formation by Concave and Convex Lenses
Images Formed by a Convex Lens-The position, size, and nature of the image formed by a convex lens can be
summarized in the table below.
Images Formed by a Concave Lens- The position, size, and nature of the image formed can be summarized in the table
Sign Convention for Lenses - The following table summarizes the sign conventions of concave and convex lenses:* A
Concave lens always forms a virtual image.
.
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Lens Formula:
f→ focal length of the spherical lens
v→ distance of the image from the optical centre
u→ distance of the object from the optical centre
Magnification
- The apparent change in the size of the object is because of the magnification produced by the lens.
-The magnification of a spherical lens gives the relative extent to which the image of an object is magnified with respect
to the object size.
-Magnification is expressed by the ratio of the image height (HI) to the object height (HO).
-Magnification (m) is also related to the object distance (u) and image distance (v) by the relation.
-If m = 1; image size = object size
-If m > 1: image size > object size
-If m < 1: image size < object size
-If magnification is positive, the image will be virtual and erect.
- If magnification is negative, the image will be real and inverted.
Power of a Lens -The ability of a lens to converge/diverge a beam of light rays is expressed in terms of its power (P).
It is the inverse of focal length, f(in meters).Power of a convex lens is positive and that of a concave lens is negative.
The SI unit of power is Dioptre(D).
1Dioptre – 1 diopter is the power of a lens whose focal length is 1meter.
Q) A ray of light is incident on a glass surface in such a way that it makes an angle of 60° with the normal (as shown in the
given figure). Determine the angle of refraction (Given that the relative refractive index, = 1.34).
( NOTE - Attach only a A4 size page along with this assignment after this page and draw all the ray
diagrams for the image formation in cases of Concave lens and Convex lens)
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Human Eye - Eye is one of the most sensitive Structure of human eye
sense organs in the human body.Our eye enables us to see this
beautiful world.
It consists of a lens, which is made up of living tissues.
The human eye is roughly spherical in shape with diameter of
about 2.3 cm. It consists of a convex
lens made up of living tissues.
Hence, human lenses are living organs contrary to the simple
optical lenses
The following table lists the main parts of the human eye and
their respective functions.
/White part of the eye
/Works as lubricant
The white of the eye is known as the sclera. It is the tough, opaque tissue that protects the outer layer of the eye. Iris is
the coloured part of the eye, and the pupil is the black, Circular hole that is located at the centre of the iris. The thin,
transparent tissue that covers the outer surface of the eye is known as the conjunctiva. It consists of tiny blood vessels
that nourish it.
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The cornea is located at the front portion of the eye. It is the transparent window that bulges outwards. The lens consists
of layers of tissues enclosed in a tough capsule. The focus of the lens is adjusted by the ciliary muscles that suspend and
hold it.
Light rays enter the eye through the cornea. The rays are bent, refracted, and focused by the cornea, lens, and the
vitreous humour. The main function of the lens is to focus the light rays sharply on the retina. It is the outer surface of
the cornea where most of the refraction of light occurs.
Iris and pupil control the size of the pupil and the amount of light respectively. Since the eye lens is convex in nature, the
resulting image is real, small, and inverted. This image is formed on the retina.
The retina converts these light rays into electrical signals with the help of light sensitive cells. These signals are sent to
the brain via translated and perceived objects in an erect or upright position.
The head of the optic nerve is devoid of photosensitive cells. Hence, no image is formed at that point called the blind
spot of the eye
Retina consists of two types of light sensitive cells – Rod cells and Cone cells. The rod shaped cells respond to the
intensity or brightness of the focused light whereas the cone shaped cells of the retina respond to the colours. Thus, the
cone cells of the retina make colour perception possible.
A person having defective cone cells is not able to distinguish between the different colours. This
defect is known as Colour Blindness.
Power of Accommodation And Defects Of Vision Of Human Eye
-To see a distant object clearly, the focal length of the lens should be larger. For this, the ciliary muscles relax to decrease
the curvature and thereby increase the focal length of the lens. Hence, the lens becomes thin. This enables you to see
the distant object clearly.
-To see the nearby objects clearly, the focal length of the lens should be shorter. For this, the ciliary muscles contract to
increase the curvature and thereby decrease the focal length of the lens. Hence, the lens becomes thick. This enables
you to see the nearby objects clearly.
-The ability of the eye lens to adjust its focal length accordingly as the object distances is called power of
accommodation.
-The minimum distance of the object by which clear distinct image can be obtained on the retina is called least distance
of distinct vision. It is equal to 25 cm for a normal eye. The focal length of the eye lens cannot be decreased below this
minimum limit of object distance.
-The far point of a normal eye is infinity. It is the farthest point up to which the eye can see objects clearly.
-The range of vision of a normal eye is from 25 cm to infinity.
Q)Why animal’s eyes are positioned on their heads?
A)This is because it provides them with the widest possible field of view. Our eyes are located in front of our face. One
eye provides 150° wide field of view while both eyes simultaneously provide 180° wide field of view. It is the importance
of the presence of two eyes as both eyes together provide the three dimensional depth in the image.
The loss of power of accommodation of an eye results in the defects of vision.
There are three defects of vision called refractive defects. They are Myopia, Hypermetropia, and Presbyopia. In this
section, we will learn about these defects of vision in detail.
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1. Myopia (short sightedness or near sightedness)
Myopia is a defect of vision in which a person clearly sees all the nearby objects, but is unable to see the distant objects
comfortably and his eye is known as a myopic eye. A myopic eye has its farpoint nearer than infinity. It forms the image
of a distant object in front of its retina.
Myopia is caused by
i. increase in curvature of the lens ii. increase in length of the eyeball
Since a concave lens has an ability to diverge incoming rays, it is used to correct this defect of vision. The image is
allowed to form at the retina by using a concave lens of suitable power.
2. Hypermetropia (Long sightedness or Far Sightedness)
Hypermetropia is a defect of vision in which a person can see distant objects clearly and distinctively, but is not able to
see nearby objects comfortably and clearly.
A Hypermetropiceye has its least distance of distinct vision greater than 25 cm.
Hypermetropia is caused due to
i. reduction in the curvature of the lens ii. decrease in the length of the eyeball
Since a convex lens has the ability to converge incoming rays, it can be used to correct this defect of vision, as you
already have seen in the animation.
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3. Presbyopia (Ageing vision defect)
Presbyopia is a common defect of vision, which generally occurs at old age. A person suffering from this type of defect of
vision cannot see nearby objects clearly and distinctively. A presbyopic eye has its near point greater than 25 cm and it
gradually increases as the eye becomes older.
Presbyopia is caused by the
i. weakening of the ciliary muscles
ii. reduction in the flexibility of the eye lens.
A person with Presbyopia cannot read letters without spectacles. It may also happen that a person suffers from both
myopia and hypermetropia. This type of defect can be corrected by using bi-focal lenses. A bifocal lens consists of both
convex lens (to correct hypermetropia) and concave lens (to correct myopia)
*It is a common misconception among people that the use of spectacles “cures” the defects of vision. However, this is
not true as spectacles only “restore” the defects of vision to the normal value.
Cataract
- It is also one of the eye defects found commonly in people of older ages. In this defect, thecrystalline lens
becomes milky and cloudy. This condition is also known as cataract. Thiscauses partial or complete loss of vision. This
loss of vision can be restored by removing thecataract by means of a cataract surgery. The use of any kind of spectacle
lenses does notprovide any help against this defect of vision.
Refraction Of Light Through A Glass Prism And Dispersion Of White Light
-When a ray of light is incident on a rectangular glass slab, after refracting through the slab, it gets displaced laterally.
As a result, the emergent ray comes out parallel to the incident ray.
-Unlike a rectangular slab, the sides of a glass prism are inclined at an angle called the angle of prism. Therefore, a
ray of light incident on its surface, after refraction, will not emerge parallel to the incident light ray (as seen in the
case of a rectangular slab).
-The extent of deviation of the light ray from its path EH to path GS is
known as the angle of deviation OR
The angle made by extending the incident with emergent ray is called
angle of deviation.
-Light bends because of refraction that takes place at points B and C
respectively, when it tries to enter and emerge from the prism
The angle of incident (angle i) increase as the angle of (D) deviation
decrease till for a particular value of angle i , angle D attends a minimum
value Dm this minimum angle of deviation is called the angle of minimum
deviation .
<i + <e = <D + <A
Dispersion Of White Light- A beam of white light will split into a
band of seven colours. The splitting of a beam of white light into its seven constituent colours, when it passes through a
glass prism, is called the dispersion of light.
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SPECTRUM- The band of the coloured components of a light beam is called its spectrum.
Isaac Newton was one of the greatest mathematicians and physicists of the world ever saw. In 1665, with the help of an
experiment he showed that white sunlight is actually a mixture of seven different colours. These constituent colours of
white light can be separated with the help of a glass prism.
Q)Why white light gets dispersed into seven colours?
A)When a beam of white light AB enters a prism, it gets refracted at point B and splits into its seven constituent colours,
viz. violet, indigo, blue, green, yellow, orange, and red. The acronym for the seven constituent colours of white light is
VIBGYOR. This splitting of the light rays occursbecause of the different angles of bending for each colour. Hence, each
colour while passing through the prism bends at different angles with respect to the incident beam. This gives rise to the
formation of the colour spectrum.
Violet- (short wavelength, deviates most)
Red- (long wavelength, deviates least)
Violet light bends the most whereas red
colour deviates least.
However, Newton did not stop at this point.
He thought that if seven colours can be
obtained from a white light beam, is it
possible to obtain white light back from the
seven colours?
For this, he placed an inverted prism in the path of a colour band. He was amazed to see that only a beam of white light
comes out from the second prism. It was at this point that Newton concluded that white light comprises of seven
component colours.
Formation of a rainbow - The rainbow is a natural
phenomenon in which white sunlight splits into beautiful
colours by waterdroplets, which remain suspended in air
after the rain. If we stand with our back towards the sun,
then we can see the spectrum of these seven colours.
Q)Why a rainbow is shaped similar to an arc?
A)This is because the rainbow is formed by the dispersion of
white light by spherical water droplets. It isthe shape of the
water droplets that gives the rainbow an arc shape.
Atmospheric Refraction
- A Star appears to twinkle because of uneven heating of atmospheric air that results in a variation in the
refractive index of air.
-Twinkling
of stars
Light coming from the stars undergoes
refraction on entering the Earth’s atmosphere.
This refraction continues until it reaches the
Earth’s surface. This happens because of
uneven heating of atmospheric air. Hence, the
atmospheric air has changing refractive index
at various altitudes. In this case, starlight
continuously travels from a rarer medium to a denser medium. Hence, it continuously bends towards the normal.
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The refractive index of air medium gradually increases with a decrease in altitude. The continuous bending of starlight
towards the normal results in a slight rise of the apparent position of the star.
Since the physical conditions of the Earth’s atmosphere keeps changing, the apparent position of the star is not
stationary. The star changes its position continuously, which makes it twinkle. This happens because starlight travels a
very large distance before reaching the observer. However, the path varies continuously because of uneven atmospheric
conditions. Hence, the stars seem to be fluctuating, sometimes appearing brighter and sometimes fainter. All this
together, gives rise to the twinkling effect of stars.
Early sunrise and delayed sunset
As viewed from the Earth, the sun rises 2 minutes before the actual sunrise and sets 2 minutes after the actual sunset.
So, we get to see sunrise 2 minutes before the actual sunrise. Similarly, after two minutes of sunset, we can still see the
sun. Hence, atmospheric refraction lengthens a day by 2 + 2 = 4 minutes every day.
We define the phenomenon of sunrise as the rise of the sun above the horizon. Similarly, sunset is defined as the
phenomenon of setting of the sun below the horizon.
Scattering Of Light
It is the phenomenon by which a beam of light is redirected in many different directions when it interacts with a particle of matter.
Tyndall effect - The Tyndall effect is caused by the scattering of light by very small air particles, which are suspended
in the Earth’s atmosphere. To observe the Tyndall effect, the particles diameter should be lessthan 1/20th of the
wavelength of the light used.
-This effect can be seen when light enters through a hole in a dark room filled with dust particles.
-The colour of the scattered light depends on the particle size. Fine particles mainly scatter blue light.
-Large particles scatter red light.
-It is observed that blue colour light scatters more easily than red colour light. This is because red colour light is of a
longer wave length.
Some natural phenomena related to the Tyndall effect
-If there was no atmosphere on Earth, there would no scattering of light. Hence, in deep space, the sky will appear to
be dark.
The least scattering red colour light finds its application in various fields. For example, in marking red light, danger signals
etc. red colour is preferred because it is scattered least by fog, smoke, and dust particles present in air.
Sunrise and sunset
At sunrise or sunset, the sun is located near the horizon of the Earth. Hence, light has to travel a long distance through
the Earth’s atmosphere. At the time of sunrise or sunset, when white sunlight falls on suspended atmospheric particles,
blue colour light scatters out in deep space, while red colour light scatters less, and reaches the observer on the surface
of the Earth. Hence, when this less scattered red light reaches our eyes, the sun and its surroundings appear to be
reddish.
Q)When located overhead, why does not the sun appear reddish in colour?
A)This is because light travels a relatively shorter distance when located overhead. At noon , the sun appears white as
Only a little of the blue and violet colours are scattered.
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*When there is no impurity present in air, the colour of the sun at sunrise and sunset appears to be
yellowish. Due to the presence of salt particles in air over seas and oceans, the colour of the sun at sunrise
or sunset appears to be orange.
*Due to the presence of red iron-rich dust, the sky appears red from the Martian surface. All these natural
phenomena take place due to the scattering of sunlight.
ASSIGNMENT PART – 1
1.Name one defect of vision which cannot be corrected by any type of spectacle lenses?
2. What is the nature of image formed on the retina of the eye ?
3. What is the function of optic nerve in human eye ?
4. Why do different colours deviate through different angles on passing through
prism?
5. What is meant by scattering of light ?
6. Name the defect of vision in person
7. What is meant by ‘total internal reflection’? State two essential conditions for
total internal reflection to take place. With the help of diagram, illustrate an
application of total internal reflection
8. How will you use two identical prisms so that a narrow beam of white light incident on one
prism emerges out of the second prism as white light? Draw the diagram
9. The given diagram shows the path of a ray of light through an equilateral prism. Answer the
following questions : (i) Which is the refracted ray? (ii) Which is the emergent ray? (iii) What name is given to angle 1?
(iv) What name is given to angle 3? (v)name is given to angle 5? (vi) What is the relation between angles 1, 2, 3 and 4?
10. Why do the faces of persons sitting opposite to you around a camp fire appear to shimmer?
11. Why does the smoke coming out of coal fired chimney appear blue on a misty day?
12. Why is red light used as universal danger signal?
13. Where do the parallel rays meet on passing through crystalline lens of :
(i) long - sighted eye, (ii) short-sighted eye?
14. What is the function of the following in human eye? (i) Yellow sport (ii) Choroid
15. Why does it take some time to see objects in a cinema hall when we just enter the hall from bright sunlight? Explain
in brief.
ASSIGNMENT PART – 2
1.A student sitting at the back of the classroom cannot read clearly the letters written on the
blackboard. What advice will a doctor give to her?
2. How are we able to see nearby and also the distant objects clearly?
3. A person needs a lens of power – 4.5 D for correction of her vision. (a) What kind of defect in vision is she suffering
from? (b) What is the focal length of the corrective lens? (c) What is the nature of the corrective lens?
4. Is the position of a star as seen by us its true position? Justify your answer.
5. Why do we see a rainbow in the sky only after rainfall?
6. A rainbow is always formed in a direction opposite to that of the sun ?
7. What is the difference in colors of the Sun observed during sunrise/sunset and noon? Give explanation for each.
8. Chicken can see only in bright light. What type of cells is present in its retina?
9. Give reason: What will be colour of the sky in the absence of atmosphere?
10.Give reason: Why are traffic light signals (or danger signals) of red colour?
11. When a light ray passes obliquely through the atmosphere in an upward direction, how does
its path generally change?
12. What is Tyndal Effect? Explain with an example.
13. The sun near the horizon appears flattened at the sun set and sun rise. Explain why.
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14. To a person the lines drawn parallel to one another appear distorted. Name the defect in eye?
15. Mixture of yellow & orange light is dispersed through prism. Colour which will deviate least?
16. What is an impure spectrum?
17. By giving reasons state your observations when a parallel beam of white light: (i) is passed through by hypo-solution
and then focused on a white screen (ii) is passed through hypo-solution (to which few drops of sulphuric acid is added)
and then focused on a white screen.
18.All NCERT INTEXT QUESTIONS .
ASSIGNMENT PART – 3
1.How is the eye lens held in its position?
2. What is meant by near point?
3. What is meant by least distance of distinct vision?
4. Which part of the eye controls the amount of the light entering the eye?
5. Which liquid fills the space behind the cornea?
6. Why is blind spot so called?7. Name the defects of vision of human eye?
8. Where is the image formed in an eye suffering from near sightedness or long sightedness?
9. How is long sightedness corrected? Draw diagram to show.
10. A person has to use a concave lens in his spectacles. What defect of vision is he suffering ?
11. What is the twinkling of stars due to?
12. Which scientist first explains the dispersion of light?
13. Name the delicate membrane in the eye having enormous number of light sensitive cells
14. What kind of lens is used in the spectacles of a person suffering from Myopia?
15. On what factor the colour of the scattered light depends?
16. What is a function of choroids?17. Why does sky appear blue on a clear sky?
18. What happens to the lens and the ciliary muscles when you are looking at nearby objects?
19. When one enters a dim-light room from bright light, one is unable to see the objects in thedim-light room for some
time. Why?
20. Why is the refractive index of atmosphere different at different altitudes?
21.ALL NCERT QUESTIONS OF LAST PAGE OF THIS CHAPTER.
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*We have tried to reduce the mistakes/errors as much as possible. But if you still find any mistakes at any point in the
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