Turnbull High School
Physics Department
CfE
National 4 /National 5
Physics
Unit 1: Waves and Radiation
Section 3: Light
Name:
Class:
1
National 5
Unit 1: Section 3
• I can state the law of reflection and describe the
reversibility of light
• I can explain the term refraction.
• I can identify the normal, angle of incidence and
angle of refraction in a refraction diagram.
• I can describe how light will behave travelling from one
medium to another and explain it in terms of changes
in wave speed.
In addition:
• I can measure the focal length of a convex lens and
calculate its power.
• I can explain the terms critical angle and total internal
reflection.
• I can describe a use for fibre optics.
• I can carry out an experiment to find the critical angle
for light in a material.
• I can draw ray diagrams for convex lenses and describe
the image formed.
• I can identify eye defects and which type of lens
should be used for sight correction.
2
Revision from S2: Light Reflection
The diagram below shows the path of a ray of light when
reflected off a mirror.
The normal is a line drawn at 90° to the mirror.
Angle of incidence = Angle of reflection
The principle of reversibility of light states that a ray of light
which travels along any particular path from some point A to another
point B travels by the same path when going from B to A, e.g. in the
above diagram the ray travels from A to O to B. If the direction was
reversed then the ray would follow B to O to A.
The Law of Reflection:-
When light is reflected the angle of incidence (measured between
the incident ray and the ________) is always ______ ___ the angle
of reflection (measured between the _________ ray and the
_________).
When a ray of light is shone back along one of the reflected rays, it
travels back along the __________ ray. This is called the
reversibility of rays of light.
3
Curved Reflectors
These can be used in transmitters and receivers of any
waves, e.g. light, sound, infrared, microwaves, TV signals
and satellite communication.
Receivers and Curved Reflectors
Fitting a c _ _ _ _ _ r _ _ _ _ _ _ _ _ dish to a receiver aerial can
make the received signal s _ _ _ _ _ _ _.
When incoming signals hit the c _ _ _ _ _ r _ _ _ _ _ _ _ _ dish, the
dish f _ _ _ _ _ _ them all onto the r _ _ _ _ _ _ _ a _ _ _ _ _ . The
r _ _ _ _ _ _ _ a _ _ _ _ _ therefore receives a s _ _ _ _ _ _ _
signal than it would if the dish was not fitted to it.
Show this by completing the diagram.
4
Refraction of Light
Your teacher will show you a short power point on refraction.
Light travels in straight lines called light r _ _ _.
_
When light passes from one material into another of
different density,
density its s _ _ _ _ changes,, its w_ _ _ _ _ _
_ _ _ and so its d _ _ _ _ _ _ _ _ could also change (unless the
light hits the material at 90° to its surface - along the normal).
normal
This change of speed of light when travelling from one material to
another is known as r _ _ _ _ _ _ _ _ _.
_
It may also cause a change in direction (bending) but not always.
A normal is a dashed line drawn at 90° to the surface of a material
where a light ray hits the material.
5
Experiment 1
Refraction of Light
In a given material (called a medium) light travels in a straight line.
When the light moves from one material to another it changes
speed and as result it may bend as it enters the new material. This
effect is called refraction.
Set up a ray box to shine a single light ray through the following
Perspex shapes. On blank paper trace round the shapes and trace
the path of the ray(s) through, and after leaving the blocks.
Complete the diagrams below (show normals and angles clearly).
(a) Plane rectangular block
Label the incident, refracted and emergent rays.
Is the direction of the ray inside the block the same as outside?
___________________________________________________
___________________________________________________
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(b) Plane rectangular block at an angle to the incident ray
Label the incident, refracted and emergent rays.
What happens to the ray when it:
(a) Enters the block ____________________________________
(b) Leaves the block
____________________________________
What do you notice about the direction of the incident and
emergent rays?
___________________________________________________
___________________________________________________
___________________________________________________
___________________________________________________
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(c) Triangular prism
Note:
___________________________________________________
___________________________________________________
Note:
___________________________________________________
___________________________________________________
8
Experiment 2
Refraction in Lenses
Set up a ray box to shine a three parallel light rays through the
following lenses - trace the path of the rays through, and after
leaving the lens.
(a)
(i) Convex lens (thin)
Do the rays converge (come together) or diverge (spread out)?
________________________________________________
What is the point where the rays meet called?
________________________________________________
Mark in the focal length.
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(ii) Convex lens (thick)
Do the rays converge or diverge?
___________________________________________________
What do you notice about the focus of the thick lens?
___________________________________________________
Mark in the focal length.
A _________ (or ___________) lens brings rays of light to a
focus. A thick lens has a ___________ focal length than a ______
lens.
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(b) Concave lens
Do the rays converge or diverge (spread out)? _______________
Focal Length and Power of Lenses
T _ _ _ _ lenses refract (b _ _ _) light more than t _ _ _ lenses
- so t _ _ _ _ lenses are more p _ _ _ _ _ _ _.
A powerful lens has a s _ _ _ _ _ _ focal length.
Convex lenses have a p _ _ _ _ _ _ _ ( __ ) power. Concave lenses
have a n _ _ _ _ _ _ _ ( __ ) power.
11
Note the power is given in dioptres (D); the focal length is measured
in metres.
Converging (convex) lenses have positive powers (e.g. +10 D, +17 D).
Diverging (concave) lenses have negative powers (e.g. –10 D, –17 D).
Example: A spherical convex lens has a focal length of 10 cm. Find
the power of the lens.
Problems:
1. A convex lens has a power of
+5 D. Calculate the length of it’s
focal length in metres.
2. A convex lens has a focal length
of 20 cm. Calculate the power of
the lens.
3. Calculate the focal length of a lens
with a power of -8D. What kind of
Lens is this?
4. Calculate the power of a concave
lens with a focal length of 40 cm
12
Experiment 3
Measuring the Focal Length of a Lens
(Parallel Rays)
What you need: Ruler, convex lenses, white paper
What to do:
1. Choose a convex lens and use it to produce a sharp image of a
distant object (e.g. a window) on the white paper.
2. Measure the distance from the lens to the image - this is the
approximate focal length.
3. Complete the table for the different lenses.
Convex lens
shape
Focal length
(cm)
X – most
curved
Y – curved
Z – least
curved
13
Power of lens
(D)
(a)
In which way do lenses with a short focal length look
different from those with a long focal length?
_____________________________________________
_____________________________________________
(b)
Does the focal length of the lens affect the image (what is
seen on the screen) in any way?
_____________________________________________
_____________________________________________
(c)
How can you tell from looking at two convex lenses which
one is the most powerful?
_____________________________________________
_____________________________________________
14
Experiment 4: Refraction
What you need: Ray-box kit; sheet of white paper; pencil.
What to do:
1. Take the semi-circular glass block and place it on a sheet of
paper and draw round it.
normal
10°
20°
30°
40°
50°
2. Send a single ray into the block as shown, then measure the angle
i, (angle of incidence), and angle r, (angle of refraction).
3. Repeat for different angles shown to complete the table.
Angle of incidence, i
(degrees)
Angle of refraction, r
(degrees)
10
20
30
40
50
15
Note:
(a) The normal is the line drawn at ____________ __________ to
the surface.
(b) The angle of incidence is the angle between the __________
______ and the ___________.
(c) The angle of refraction is the angle between the ____________
______ and the __________.
(d) How does the angle of incidence compare with the angle of
refraction?__________________________________________
___________________________________________________
(e) How would the angles of incidence and refraction compare if no
refraction took place?
___________________________________________________
___________________________________________________
(f) How would the angles of incidence and refraction compare if the
block was made from a material which caused more refraction?
___________________________________________________
___________________________________________________
___________________________________________________
(g) When a ray of light is sent into the centre of the straight edge
of a semicircular glass block (as above) there is no change in
direction when exiting from the curved edge. Can you explain this?
___________________________________________________
___________________________________________________
___________________________________________________
___________________________________________________
16
INVESTIGATION
Experiment 5: Total internal reflection
Aim: To investigate total internal reflection and measure
the critical angle for Perspex.
Apparatus: Semicircular Perspex block, a protractor, a ray box and
power supply. Collect an Investigation Booklet.
Instructions:
• State the aim of the experiment.
• Set up the apparatus as shown.
• Slowly increase the angle of incidence.
• Describe what happens.
• By drawing a ray on your diagram when the angle of refraction is
90°, estimate the critical angle for the Perspex block
17
Critical Angle and Total Internal Reflection
As the angle of incidence inside the block increases the angle in air
increases. Beyond a certain angle the ray is no longer refracted out
into the air. When no light passes from glass to air we have what is
called Total Internal Reflection. The smallest angle of incidence at
which total internal reflection takes place is called the Critical
Angle. Different transparent materials have different critical
angles. At angles greater than the critical angle all the light is
reflected back inside the block!
The critical angle for a material can be found using the
experimental procedure described in experiment 5.
18
Summary of Refraction
(a) Refraction takes place when light goes from one material
(medium) to another. This causes the speed of the light to
change.
(b) When light goes from air to glass it:
slows down and is bent towards the normal.
AIR
GLASS
Incident Ray
Refracted Ray
Normal
(c) When light goes from glass to air it:
speeds up and is bent away from the normal.
GLASS
AIR
Refracted Ray
Incident Ray
19
Optical fibres
An optical fibre has a dense solid glass core surrounded by a
less dense solid glass coating. Unlike a mirror, there is not a
"silvered surface" in the optical fibre. When a light ray hits the
boundary between the core and coating at an angle greater than the
C _ _ _ _ _ _ _ A _ _ _ _, all of the light ray is
reflected back into the optical fibre - This is known
as T _ _ _ _ I _ _ _ _ _ _ _ R _ _ _ _ _ _ _ _ _.
Optical fibres are used in medicine and telecommunications.
Complete the diagram below:
Optical fibres are often used in preference to copper wire in
communications systems because they are:
• Cheaper to produce
• More lightweight and flexible
• Able to carry more signals per fibre and the signals are free
from electrical interference.
• There is little loss of energy due to the pure glass.
20
Experiment 6: Optical Fibres
Aim: To demonstrate light transmission through an optical
fibre.
Apparatus: A radio, an LED transmitter, a photodiode receiver, an
amplifier, optical fibre and a 12 V light bulb with power supply.
Instructions
• Hold one end of the optical fibre near a light bulb and view the
other end.
• Set up the apparatus as shown above.
• Explain, using a diagram, how the light signal is carried through the
fibre.
___________________________________________________
___________________________________________________
___________________________________________________
___________________________________________________
21
The Fibrescope (Endoscope)
In one important development of this principle - fibre optics flexible bundles of fine glass fibres transmit light round corners
with ease. As each fibre is coated and transmits light independently
of the others it is possible to 'see' through or take photographs
through such a system. This principle is used to enable doctors to
inspect inaccessible parts of the body - e.g. the stomach.
Each individual fibre consists of a central core of high optical glass
coated with a thin layer or cladding of another glass. This cladding
prevents the light, which enters the end of the fibre, from escaping
or passing through the sides to another fibre in the bundle. Light
travels din the fibres using total internal reflection.
The bundle used to carry the light from the external light source
to shine on the object is called the light guide bundle. The bundle
called the image guide is used to carry the image (picture) back to
the eye.
22
The heat from the lamp does not pass down the fibres. This
means that the other end of the guide is cold (called a cold light
source). This is safer for the patient.
Fibrescopes usually have a controllable bending section near the tip
so the observer can direct the scope during insertion and be able
to scan an area once inside.
1. Draw a simple diagram of an optical fibre to show total internal
reflection.
2. Why is the end of the fibrescope 'cold' and explain why this is
useful?_________________________________________
_______________________________________________
_______________________________________________
3. Which parts of the body do doctors study using an endoscope
(fibrescope)?_____________________________________
_______________________________________________
4. Explain why two separate bundles of fibres are used in the
fibrescope?______________________________________
_______________________________________________
_______________________________________________
5. Why do you think that fibrescopes have a controllable, flexible
bending section at the tip?
_______________________________________________
______________________________________________
23
Tutorial 1
1. a) What is the law of reflection?
_____________________________________________
_____________________________________________
b) Complete the diagram below to illustrate your answer to
part (a). You must label the angle of incidence (i), the
angle of reflection (r) and the normal on your completed
diagram.
mirror
2. a) i) Explain what is meant by the term refraction.
_______________________________________________
_______________________________________________
ii) Name two examples of applications which make use of
this refraction.
_______________________________________________
_______________________________________________
b) Complete the ray diagram to show light entering and leaving
the glass block.
Label the diagram showing:
24
i)
An angle of incidence (i)
ii) An angle of refraction (r)
iii) A normal
3. Copy and complete each of the following diagrams to show
what happens to the ray of light as it passes from air into
glass.
25
4. Light can be transmitted along an optical fibre through the
process of total internal reflection.
Complete the diagram below to show how the signal is
transmitted along the optical fibre.
5. Doctors can use an endoscope to examine internal organs of a
patient.
The endoscope has two separate bundles of optical fibres that
are flexible.
a) Explain the purpose of each bundle of optical fibres in the
endoscope.___________________________________
_____________________________________________
___________________________________________
b) The tip of the endoscope that is inside the patient is
designed to be very flexible. Suggest one reason for this.
_____________________________________________
___________________________________________
26
Image Formation on the Retina
Your teacher will show you a model eye.
At the back of an eye, there is a layer of light-sensitive cells called
the r _ _ _ _ _.
When we look at an object, an image (picture) of the object is
formed on the r _ _ _ _ _.
The image is u _ _ _ _ _ d _ _ _ and l _ _ _ _ _ _ _ _
i _ _ _ _ _ _ _ ( b _ _ _ to f _ _ _ _ ).
27
Looking at Distant Objects
Complete the diagram below:
When we look at an object some distance from the eye, the light
rays from the object which enter our eye are p _ _ _ _ _ _ _ to
one another.
The muscles around our eye lens are r _ _ _ _ _ _, so the eye lens
is t _ _ _.
Looking at Close Objects
Complete the diagram below:
When we look at an object close to the eye, the light rays from the
object which enter our eye are n _ _ p _ _ _ _ _ _ _ to one
another.
The muscles around our eye lens squash it, making the lens
t _ _ _ _ so it can focus the light rays on the retina.
28
Ray Diagrams
Image formation by a converging lens
Images can be described as:
• real (light goes through the lens and image can be seen on screen)
or virtual (we cannot get this image on a screen).
• inverted or upright
• magnified, same size or diminished (smaller).
We can draw ray diagrams to determine the nature of the image
formed by a lens for an object a particular distance away.
Drawing Ray Diagrams
• Choose an appropriate scale (better done on graph paper).
• Draw ray 1 from the tip of the object parallel to the axis,
passing through the focal point of the lens.
• Draw ray 2 from the tip of the object, passing through the
centre of the lens.
• Where the two rays meet will be the tip of the image of the
object.
Object distance greater than twice the focal length
29
Object distance less than the focal length
These two rays do not meet on the right hand side, there is no real
image.
However, we can extend the lines to the left to make them meet.
The image is drawn where these two lines meet. This is a virtual
image. (dashed lines show this). The light does not go backwards to
the left of the lens! If we look through the lens from the right we
will see this virtual image “floating in space”. We cannot get this
image on a screen.
30
Experiment 7: Investigation of Image Formation
Aim: To see how the position, nature and size of an image
depends on the object distance from a converging lens.
Apparatus: Converging lens of known focal length, lens holder,
illuminated object, metre stick and a white screen, Graph paper.
Instructions
• Set up the apparatus as shown above.
• Position the object distance at greater than twice the focal length
of the lens.
• Move the screen until a clear image is formed on it.
• Note the type of image formed and the image distance from the
lens.
• Repeat the procedure for the following object distances and
complete the table:
(a) Exactly two focal lengths
(b) Between one and two focal lengths
(c)
Less than one focal length.
Object position from lens
Type of image
More than 2xFocal length
Exactly 2x focal length
Between one and two focal lengths
Less than one focal length
• Check your answers by constructing ray diagrams for each case.
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32
A person who is l _ _ _ s _ _ _ _ _ _can see c _ _ _ _ _ _
objects which are f _ _ a _ _ _ - This is because the eye
c _ _ focus the p _ _ _ _ _ _ _ light rays coming from the
object on the r _ _ _ _ _.
However, the person cannot see c _ _ _ _ _ _ objects which
are c _ _ _ _ to them - This is because the eye c _ _ _ _ _
focus the n _ _ - p _ _ _ _ _ _ _ light rays coming from the
object on the r _ _ _ _ _.
Complete this diagram to show how a
“long-sighted” eye focuses light rays
from a close object
To correct long-sight a c_ _ _ _ _ l_ _ _ is
placed in front of the eye.Complete this
diagram to show the affect the lens has on
light rays from a close object.
33
34
A person who is s _ _ _ _ s _ _ _ _ _ _ can see c _ _ _ _ _ _
objects which are c _ _ _ _ - This is because the eye
c _ _ focus the n _ _ - p _ _ _ _ _ _ _ light rays coming from
the object on the r _ _ _ _ _.
However, the person cannot see c _ _ _ _ _ _ objects which
are d _ _ _ _ _ _ (f _ _ a _ _ _ ) - This is because the eye
c _ _ _ _ _ focus the p _ _ _ _ _ _ _ light rays coming from the
object on the r _ _ _ _ _.
Complete this diagram to show how a
“short sighted” eye focuses light rays from a
distant object
To correct short sight a c_ _ _ _ _ _ L_ _ _is placed
in front of the eye. Complete the diagram to show
the affect the lens has on light rays coming from a
distant object.
35
Experiment 8: The model eye
(Teacher Demonstration)
Aim: To demonstrate how short and long sightedness can be
remedied using lenses.
Apparatus:
Flask filled with fluoroscene
Ray box
Model of Retina
Instructions
• With the rays focusing short of the retina, place a various lenses
in front of the flask until they focus on the retina.
• Repeat with the rays focusing behind the retina.
Short sightedness is corrected by wearing glasses with _________
lenses.
Long sightedness is corrected by wearing glasses with __________
lenses.
36
Tutorial 2
1. Complete the diagram below to show how a healthy eye
would focus parallel rays of light.
retina
2. Copy and complete the table below to give information
about short and long sight.
Eye Defect
Description
Lens Used
Short-sightedness
Long-sightedness
3. Draw a convex lens, and show how it affects parallel
rays of light.
4. Draw a concave lens, and show how it affects parallel
rays of light.
37
5. Figure 1 shows light rays entering the eye of a longsighted pupil.
a) Copy and complete the diagram below to show how the light
rays reach the retina of this long-sighted eye.
retina
Figure 1
b) Complete the diagram below and, in the dotted box in Figure
2, draw the shape of the lens that would be used to correct
this eye defect.
retina
Figure 2
c) On your diagram of Figure 2 complete the path of the
rays from this lens until they reach the retina.
d) When this sight defect has been corrected, the student
looks at a picture in a text book. How does the image on
the retina of the student’s eye compare to the actual
picture? __________________________________
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6. A girl wants to find the focal length of a convex lens she
has found in the classroom.
(a)
Describe how the focal length of the lens can be
found experimentally.
Your description should include:
(i) A list of apparatus used
(ii) A description of the procedure
_______________________________________________
_______________________________________________
_______________________________________________
_______________________________________________
_______________________________________________
_______________________________________________
b) Why does the girl use light from the window rather than
from the classroom lights?
___________________________________________
c) The girl finds the focal length of the lens to be 18 cm.
Calculate the power of the lens.
d) The girl finds a second lens which is labelled with a power of
– 20 D. Name this type of
lens._____________________________________
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7. A coin which is 1·5 cm high is held near to a lens whose
focal length is 3 cm.
← 3 cm →
1·5 cm
axis
Describe the image produced when the coin is held:
( a)
4 cm from the lens. ______________________
(b)
6 cm from the lens. _______________________
(c)
8 cm from the lens. _______________________
(Hint: You must draw separate ray diagrams on graph paper for each
problem!)
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Unit 1: Section 3 - Additional notes
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Unit 1: Section 3 - Additional notes
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