ISLAMABAD ACADEMY
PHYSICS FOR 10TH CLASS (UNIT # 14)
SHORT QUESTIONS
Define Reflection of light. When light strikes to any hard surface it bounced back in its original direction.
This bouncing back of light from any surface is called reflection of light.
Define Laws of Reflection
Reflection of light obeys following laws: i) The angle of incidence (i) is equal to the angle of reflection (r).
ii) The incident ray, reflected ray and normal at the point of incidence, all lies in the same plane.
What are spherical mirrors?
Spherical mirror in fact is a portion of reflecting surface of a hollow sphere.
What are the types of spherical mirrors?
Concave Mirror:
Mirror whose inner curved surface is reflecting is called concave mirror. It is thinner at the center and
thicker at the edges.
Convex Mirror:
Mirror whose outer curved surface is reflecting is called convex mirror. It is thicker at the center and
thinner at the edges.
Define Center of Curvature:
The centre C of the sphere, of which a concave mirror or convex mirror is a part is known as the center
of curvature.
Define Radius of Curvature:
Radius of the sphere is called Radius of Curvature.
Define Aperture:
The front end of the spherical mirror is circular and its diameters is known as Aperture.
Principal Axis:
The center P of the mirror is called the pole of mirror. A line joining the pole P of the mirror and C, is
called Principal Axis.
Define Principal Focus:
In concave mirror rays of light parallel to the principal axis after reflection converge to a point F. This
point is called Principal Focus. In convex mirror, rays parallel to the principal axis after reflection appear
to come from a point F situated behind the mirror. This point is called Principal Focus.
Define Focal Length:
The distance between the pole and the principal focus of a spherical mirror is called the Focal Length. It
is denoted by f. Radius of curvature of a spherical mirror is twice of its focal length.
Define Linear Magnification
The ratio of the height of the image to that of the object is called linear magnification. It is denoted by
‘m’.
Image Height
Magnification = m = ----------------------Object Height
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Image height q
m = --------------- = Object height p
Which mirrors are used in huge shopping centers and why?
Convex mirrors are used for security purposes in shopping centers.
Why concave mirror is used for shave?
Because when a man stands between the principal focus and pole of a concave mirror, he sees and
enlarged erect and virtual image of his face.
Define Refraction of Light
When light enters into a new medium from another medium it deviates from its original path. This
deviation is called refraction of light.
Define Laws of Refraction of Light
1. Incident ray, refracted ray and the normal all lie in same plane.
2. When a ray of light passes from one particular medium to another, the ratio of the sine of the angle of
incidence (i) to the sine of angle of refraction (r) is constant. This constant ratio is called refractive index
of the second medium with respect to the first and it is denoted by the letter (n).
What is Total Internal Reflection
When the value of angle of incidence becomes greater than the critical angle then the ray does not pass
into the second medium (air) that is the ray of light no longer suffers refraction but all the rays having
angle of incidence greater than the critical angle are totally reflected back in the denser medium (glass)
obeying the laws of reflection. Such a reflection of light is called “Total Internal Reflection”.
What are the conditions necessary for Total Internal Reflection.
The two conditions are necessary for total internal reflection: i) The ray of light should travel from a denser medium to a rarer medium.
ii) The angel of incidence should be grater than the critical angle.
Define critical angle?
The angle of incidence in the denser medium for which corresponding angle of refraction is 900 in the
rarer medium is called the critical angle. This angle of incidence is denoted by C.
What should be the angle of incidence for total internal reflection?
For total internal reflection, the angle of incidence should be greater than the critical angle.
Why in hot summer season images of cars are seen on roads and images of aeroplanes are seen on
runways?
This is due to total internal reflection.
What are totally reflecting prism?
A totally reflecting prism is that whose one angle is of 90o and remaining two angles are of 45o each.
This prism is used to erect the image of an object. i.e. to invert the rays coming from an object.
What is principal of periscope? What is its structure and uses?
Periscope works on the principal of total internal reflection. It has two adjusted totally reflecting prisms.
With the help of periscope
i) Commander of submarine can see the objects over the surface of water.
ii) Tank driver can see what is happening outside.
Define Optical Fibre.
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Optical fibre works on the principal of total internal reflection. An optical fibre is made of a highly
transparent fine strand of glass or plastic coated whose refractive index is less than the inner tube.
Which sign conventions are used in case of lenses?
i) All the distances are measured from the optical centre of lens.
ii) The distance of real images and real objects are taken positive while distance of virtual images and
virtual objects are taken negative.
iii) The focal length is taken as positive for convex lens and negative for concave lens.
Which lens has greater power, one of short focal length or of greater focal length?
Length of a lens is f then its reciprocal 1/f is called power of a lens. Therefore the lens of smaller focal
length has greater power.
To get a greater and clear image at what distance should the lens be placed from the object?
For a greater and clear image the lens should be placed from the object at a distance equal to its focal
length.
Define power of a lens? What is its unit?
If the focal length of a lens is f then its reciprocal 1/f is called the power of the lens. So
Power of lens = p = 1/f The unit of power of lens is Dioptre. Dioptre is defined as the power of a lens of
focal length of one meter. It id denoted by D.
Why a pond of clear water appears to be shallow than it really is?
This is due to refraction of light.
What will be the form of Snell’s law when light enters from glass into the air?
Speed of light in glass is 2 x 108 m/s and that in air is approximately 3x108 m/s. Therefore
Refractive Index of glass with respect to air
Speed of light in air
= -------------------Speed of light in glass
3x108 m/s
n = -----------2x108 m/s
What will happen when angle of incidence becomes more than the critical angle when light enters
from denser medium to rare medium?
Then the rays does not pass into the second medium (air) that is the ray of light no longer suffers
refraction but all the rays having angle of incidence greater than the critical angel are totally reflected
bank in denser medium (glass) obeying the laws of reflection. This type of reflection is called Total
internal reflection.
In hot summer days reflections of motor cars are seen on the roads and image of an aeroplane is seen
on runway the why?
It is due to total internal reflection it is known as mirage.
How optical fibre is made?
An optical fibre is made of a highly transparent fine strand of glass or plastic coated whose refractive
index is less than the inner tube
Why light rays bends in refraction?
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Because value of refractive index is different in different mediums. So when the medium changes, rays
bends towards or away the normal due to difference in refractive index.
Why some animals like fish move their eye lenses?
By moving their lenses, they are able to see clearly objects around them.
What do you mean by regular and irregular reflection?
When light rays reflect after striking a smooth surface, it reflect back making the angle equal to
incidence angle. This type of reflection is called regular reflections. When light rays reflect after striking
a rough or non-smooth surface, it reflect back making un-certain angles in different directions. This type
of reflection is called irregular reflections.
*** Draw figures.
What do you mean by spectrum?
After passing through prism, sunlight splits into its basic 7 colours. The band of colours which is seen on
the screen is called solar spectrum.
What is the arrangement of colours in primary rainbow?
The order is red, orange, yellow, green, blue, indigo and violet.
LONG QUESTIONS
>> Question: Derive the Concave Mirror Formula?
Consider the following figure:
i) An object OA is placed in front of concave mirror.
ii) A ray from point A, strike the mirror at point E.
iii) After reflection, it passes through the principal focus ‘F’.
iv) A second ray also starting from A falls on mirror at pole P.
v) It is reflected at an angle equal to incidence angle and intersect the first reflected ray at point B.
vi) Thus point B is the real image of point A.
vii) If this process is repeated for other points of the object OA then a real image IB of the object OA is
obtained.
. APO and . IPB are right angled triangles and are similar, therefore (see in book)
Distance of object from mirror id denoted by p and of image is q. And focal length of the mirror is
denoted by f.(see in book)
Therefore equation can be written as
>> Question: Derive the Convex Mirror Formula?
Consider the following figure:
i) An object OA is placed in front of the convex mirror.
ii) A ray of light from point A, moving parallel and strike the mirror at point E.
iii) It is reflected in the direction EM.
iv) If this line EM is produced backward (in dotted line), it meet the principal axis at principal focus F.
v) This ray appears to diverge from F.
vi) Another ray from point A, falls on the pole P and reflected at an angle equal to angle of incidence.
vii) If this ray PR is produced backward (in dotted line), it intersect the first ray at point B.
viii) Thus point B is the virtual image of A.
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ix) If this process repeated from other points of object OA then the image IB of the object is obtained.
x) This image is virtual, erect and diminished. Using the above figure, we can prove that the relationship
between the object distance p, form the pole, the image distance q from the pole and the focal length of
convex mirror f is given below: (see in book)
Distance of virtual image and focal length of convex mirror is taken as negative always.
>> Question: Write some uses of spherical mirrors?
The uses of convex mirrors are as follows:
i) Doctors use concave mirrors to examine ear, nose, throat and eye.
ii) Parabolic shaped concave mirrors are used in search lights to throw light at a larger distance.
iii) Concave mirrors are also used as shaving mirrors because it produce enlarged erect and virtual
image.
iv) Concave mirrors are used to throw light on slides in microscopes.
v) Developed countries are using concave mirrors in telescopes.
vi) Convex mirrors are used in auto mobiles to see behind and around.
>> Question: What is refractive index? How is it calculated?
When a ray of light passes from one particular medium to another, the ratio of the sine of the angle of
incidence (i) to the sine of angle of refraction (r) is constant. This constant ratio is called refractive index
of the second medium with respect to the first and it is denoted by the letter (n).
Sin i
n = ---Sin r
It is called Snell’s law. For a ray of light entering the second medium perpendicularly through the surface
of separation and in this case the ray of light shows no change of direction. The refractive index of a
medium can be calculated by dividing the speed of light in vacuum by speed of light in that medium. For
example
Speed of light in air
Refractive index of glass with respect to air = -----------------------Speed of light in glass
>> Question: Explain the mechanism of reflection through a prism?
*** See figure from book.
i) Prism is transparent body having three rectangular and two triangular surfaces as show in figure
below:
ii) A ray PQ strikes the surface AB.
iii) On entering it bends toward normal but on leaving it bends away from normal again.
iv) The original direction of ray is PQT, but it is turned through angle TDS on passing through prism.
v) The angel TDS is known as angle of deviation. It depends upon incidence angle.
vi) The minimum value of the angle of deviation is known as the angle of minimum deviation and is
denoted by D.
vii) The refraction of waves depends upon wavelength.
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viii) Therefore after passing through prism, sunlight splits into its basic 7 colours.
ix) The band of colours which is seen on the screen is called solar spectrum.
>> Question: What are optical fibers. Write its some applications and their uses?
Optical fibre works on the principal of total internal reflection. An optical fibre is made of a highly
transparent fine strand of glass or plastic coated whose refractive index is less than the inner tube. Its
applications are as under: Light Pipes:
i) If many thousands of optical fibres are bundled together than a flexible light pipe is obtained.
ii) It is used by doctors and engineers to examine things and places.
iii) Pakistan is using optical fibres in telecommunication. A single strand of optical fibre can transmit
thousands of phone calls without interfering them.
iv) Generally a cable consists of 30 optical fibres.
Endoscope:
i) It is used for viewing and photographing of internal structure of human body.
ii) It consists of bundles of many optical fibres.
iii) Endoscope consists of many instruments i.e. Gastroscope is used to examine stomach, Bronchoscope
is used to examine throat, and cystoscope is used to examine liver.
>> Question: What is rainbow? Also describe its types.
Sometimes an arc of spectral colours appears in the sky after a rainfall. This is called rainbow. It is
produced due to dispersion of light. After rain tiny droplets of water remain suspended in air. When sun
light falls on these droplets they behave like a prism and split the light into its basic colours and in this
way we see rainbow. It is has two types: Primary Rainbow:
i) The rainbow with brightest colours and can easily visible is called primary rainbow.
ii) In this case light suffers only one internal reflection in rain drops.
iii) In this rainbow red is outer colour and violet is inner colour.
iv) The order is red, orange, yellow, green, blue, indigo and violet.
Secondary Rainbow:
i) It is seen sometimes beyond the outer edge of primary rainbow.
ii) It is fainter and seldom visible.
iii) This is produced when ray from the sun suffer double internal reflection.
iv) In this rainbow violet is outer colour and red is inner colour.
v) The order of colours is reverse to primary rainbow.
>> Question: Derive the convex lens formula?
Consider the following figure:
i) An object OA, is placed in front of a thin convex lens.
ii) A ray of light starting from point ‘A’, moving parallel to the principal axis strikes the lens at the point
‘E’.
iii) After refraction through the lens, it passes through the principal pocus F.
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iv) A second ray AC also starting from ‘A’ passes through the optical centre of the lens and moves
straight (undeviated) and intersects the first refracted ray at the point ‘B’.
v) Thus ‘B’ is the real image of point ‘A’.
vi) If this process is repeated for other points of the objects OA then a real image IB of the object OA is
obtained. (see in book)
vii) Distance of the object from the lens is represented by p and that of image by q.
>> Question: Derive the concave lens formula?
Consider the following figure:
i) An object OA is placed in front of a thin concave lens.
ii) A ray of light starting from the end point A and moving parallel to the principal axis strikes the lens at
the point E.
iii) This ray is refracted along ER.
iv) A second ray AC also starting from A passes through the optical centre of the lens and is refracted
straight along its own path.
v) When the ray RE is produce backwards, it appears to intersect the first ray at the point b.
vi) Thus B is the virtual image of A.
vii) If this process is repeated for the other pints of the object OA then the virtual image IB of the object
OA is obtained. This image is virtual erect and diminished. Since a concave lens always gives virtual
image irrespective of the distance of the object from it. Therefore according to sign conventions, the
distance of the virtual image is taken as negative and that of focal length is also taken as negative.
Therefore by putting negative sign for q and f we get,
>> Question: Write a note on defects of lens?
There are two types of defects of lens which are: Spherical Aberration:
Rays which pass through thick lenses, or lenses of large aperture do not focus at a single point. Thus the
image formed are not sharp and well defined. This defect is called spherical aberration. It can be
minimized by two methods:
i) In cheaper instruments, a disc of size equal to lens is placed. A small hole is present in disc which
allows central rays to pass forming a sharp image.
ii) In expensive instruments, this defect is removed by using a complicated lens made by combining
lenses of different shapes.
Chromatic Aberration:
Sometime we notice that an object illuminated by white light and viewed through a cheap convex lens
shows coloured tinge in image. This makes the image blurred. This defect is called chromatic aberration.
This defect can be minimized by combining a convex lens of crown glass and a concave lens of flint glass
in such a way that dispersion of light produced by the convex lens is neutralized by concave lens. This
combination of lens is called “Achromatic Lens”
>> Question: Write a note on defects of vision?
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A normal person can see at a far distance down to about 25cm from the eye. There are two main
defects of eye: Short Sightedness:
i) A man suffering with short sightedness cannot see distant object clearly.
ii) This is due to thickness of eyeball than normal or eye ball is longer than normal size.
iii) In this case image is formed in front of retina instead of retina.
iv) It can be minimized by wearing spectacle having suitable concave lens.
v) Concave lens diverge the light rays and focused them on retina.
Long Sightedness:
i) A man suffering with long sightedness cannot see nearer objects clearly.
ii) This is due to thinness of eyeball than normal or eye ball is shorter than normal size.
iii) In this case image is formed behind the retina instead of retina.
iv) It can be minimized by wearing spectacle having suitable convex lens.
v) Convex lens converge the light rays and focused them on retina.
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