5.0 light

Physics Module Form 4 Chapter 5 - Light GCKL 2011

5.1
UNDERSTANDING REFLECTION OF LIGHT

What light is? Is a form energy. Light travel in a straight line and high speed about 300,000 km s-1.

How the light 1. The light ray that strikes the surface of
ray reflected the mirror is called incident ray.
by the surface 2. The light ray that bounces off from the
of mirror? surface of the mirror is called reflected ray.
3. The normal is a line perpendicular to
the mirror surface where the reflection
occurs.
4. The angle between the incident ray and
the normal is called the angle of incidence
AO = incident ray
,i.
OB = reflected ray
5. The angle between the reflected ray and
i = angle of incident
the normal is called the angle of
r = angle of reflected
reflection, r.

What is the The Laws Of Reflection
Law of 1. The incident ray, the reflected ray and the normal all lie in the same plane The angle of
Reflection ? incident, i, is ….equal….. to the angle of reflection, r.

Draw the ray
diagram of the 1. Consider an object O placed in front of a
plane mirror plane mirror.
2. Measure the distance between the object
o and the mirror.
3. Measure the same distance behind the
mirror and mark the position as the image.
4. Draw the diverging ray from a point on
the image to the corner of the eye. The rays
from the image to the mirror must be
dotted to show that are virtual.
5. Finally, draw two diverging rays from
the object to the mirror to meet the
diverging rays from the image.

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State the 1 laterally inverted
characteristics 2. same size as the object
of the image 3. virtual
formed by 4. upright
plane mirror 5.distance between image and mirror same
as distance between object and mirror.

What is meant
by virtual
image? Image that …cannot………. be seen on a screen.

What is meant
by real image?
Image that …...can…be seen on a screen.

CURVED MIRRORS:
Concave mirror Convex mirror

f f
r r

State the 1.Light (diverged, converged) 1.Light (diverged, converged)
differences 2.(virtual,real) principal focus 2. (virtual,real) principal focus
between 3.PF = Focal length 3. PF= ….Focal length…
concave mirror = Distance between the virtual = Distance between the real principal focus
and convex principal focus and the pole of the and the pole of the mirror.
mirror mirror.

Common Refer to the diagrams above and give the names for the following:
terminology of
reflection of 1.Centre of curvature ,C = The geometric centre of a hollow sphere of which the concave or
light on a convex mirror is a part.
curved mirror 2.Pole of mirror, P = The centre point on the curved mirror
3.Radius of curvature ,r = CP = radius of the curvature
4.Focal length, f = The distance between the principle focus, F and the pole of the mirror, P
5.Object distance, u = Distance of object from the pole of the mirror, P
6.Object distance , v = Distance of image from the pole of the mirror,P

Construction
Rules for
Concave
Mirror

Ray 1 Ray 2 Ray 3

5-2


A ray parallel to the A ray through F is reflected A ray through C is reflected
principle axis is reflected to parallel to the principle back along its own path.
pass through F. axis.
Image formed Using the principles of construction of ray diagram, complete the ray diagrams for each of the
by concave cases shown below:
mirror:
u = object distance; v = image distance ; f = focal length ; r = radius of curvature

Note: Point of intersection in the position of the image

A u < f ( Object between F and P ) Characteristics of Application:
image: 1.magnifying mirror
1.virtual 2.sharing mirror
2.upright 3. make-up mirror
3.magnified

B u = f ( Object, O is at F ) Characteristics of Application:
image: A reflector to
1.Image at infinity produce parallel
beam of light
such as a reflector in
1. torchlight
2.spotlight

C f < u < 2f or f < u < r ( Object O is Characteristics of
between F and C image:
1.magnified
2.real
3.inverted

I

D u = 2f or u = r ( Object ,O is at C) Characteristics of Application:
image: 1.reflector in a
1.same size projector
2.real
3.inverted

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Eu > 2f or u > r ( Object, O is beyond C ) Characteristics of
image:
1.diminished
2.real
3.inverted

I

F u =  ( Object ,O very far from the lens) Characteristics of Application:
image: Used to view distant
1.diminished objects as in a
2.real reflecting telescope
3.inverted

I

Construction
Rules for
Concave
Mirror

Ray 1 Ray 2 Ray 3
A ray parallel to the A ray towards F is reflected A ray towards C is reflected
principal axis is reflected as parallel to the principal back along its own path.
if it came from F. axis.

Image formed Using the principles of construction of ray diagram, complete the ray diagrams for each of the
by concave cases shown below:
mirror:
u = object distance; v = image distance ; f = focal length ; r = radius of curvature

image: 1. Blind Conner
1.diminished mirror
2.virtual 2.Wide side view
3.upright mirror

5-4


Check Yourself:
Objective Question:

1. Which of the following is true of the laws of 4. A boy stands in front of a plane mirror a distance
reflection f light? 5 m . When the boy moves toward the mirror by 2
A The angle of incident is equal to the m , what is the distance between the boy and his
angle of refraction new image?
B The incident ray and the reflected ray
are always perpendicular to each A 2m B 4m
other. C 6m D 8m
C The incident ray , the reflected ray E 10 m
and the normal line through the point
of incidence, all lie on the same plane.
5. An object is placed in front of a plane mirror.
2. The diagram shows a single ray of light being Compare to the object, the image formed in the
directed at a plane mirror. mirror is always

A virtual
B smaller
40° C bigger
D three times as far away

What are the angles of incidence and reflection?
Angle of incidence Angle of 6. A light ray incident onto a plane mirror at an
reflection angle of 50o
A 40o 40o The characteristics of an image , formed by a
B 40o 50o convex mirror for all positions of the object are
C 50o 40o A diminished, real and inverted
D 50 o
50o B magnified , real, and upright
C diminished ,virtual and upright
3. The diagram shows a ray of light from a small bulb D magnified , virtual and inverted
strikes a plane mirror.

7. A concave mirror has a focal length 20 cm.
What happen to the size of image when an object
is placed at a distance of 40 cm in front of the
mirror?

A diminished
B magnified
C same size of object

Where is the image of the bulb formed and its
characteristic?
A At P and virtual
B At Q and real
C At R and virtual

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8. The figure shows a candle placed in front of a Section A (Paper 2)
concave mirror of focal length, f.
Structure Question:

1. Diagram 3.1 shows a mirror at the corner of a shop.

The image formed is
A real, upright and magnified
B real, inverted and diminished DIAGRAM 3.1 / RAJAH 3.1
C virtual, inverted and magnified
D virtual, upright and diminished (a) Name the type of mirror shown in Diagram 3.1
Convex mirror
……………………………………………………..
9. When an object is placed at a point 20 cm in [1 mark]
front of a concave mirror, a real image of the
same as the object is formed on a screen placed (b) Name one characteristic of the image formed
next to the object. What is the focal length of the by the mirror.
mirror? Upright / diminished / smaller / virtual
……………………………………………………..
A 5 cm
[1 mark]
B 10 cm
(c) Sketch a ray diagram to show how the image
C 15 cm
is formed.
D 20 cm
1. Draw a parallel ray from the object that is
10. Which of the following states the right reason for incident along a path parallel to the principal axis
replacing a plane mirror are used as rear- view appears to go through the focal point
mirrors in motor vehicles with a convex mirror ?
2. A radial ray that is incident through the centre
A To shine the object of curvature, C of the curved mirror is reflected
B To widen the field of view back along the incident path through point C
C To produce a brighter image
D To produce a sharper image 3. Determine the correct position of the image

Answer:

1 C
2 D
3 A
4 C [3 marks]
5 A (d) What is the advantage of using this type of
6 C mirror in the shop?
7 A To increase the field of vision
……………………………………………………………
8 A
[1 mark]
9 B
10 C
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2. Diagram 4.1 shows the image of a patient’s teeth Section B(Paper 2)
seen in a mirror used by a dentist.
Essay Question(20 marks)

Diagram 7.1 shows two cars, P and Q , travelling in
the opposite directions, passing through a sharp band.
A mirror is placed at X .

DIAGRAM 4.1

(a) Name the type of the mirror used by the
dentist.

Concave mirror
………………………………………… …….
[ 1 mark ]

(b) State the light phenomenon that causes the DIAGAM 7.1
image of the teeth
(a) Diagram 7.2 shows an incomplete ray
Reflection of light diagram when a plane mirror is placed at X.
………………………………………………..........
[ 1 mark ]

(c) State two characteristics of the image
formed.

Virtual, upright and magnified
…………………………………………………….
[ 2 marks ]

(d) In the diagram below, the arrow represents
DIAGRAM 7.2
the teeth as the object of the mirror.
Complete the ray diagram by drawing the (i) Complete the ray diagram in Diagram 7.2
required rays to locate the position of
the image. [2 marks]

1. Two reflected rays are shown (diagram)[1 mark]
2. Angle of incidence = Angle of reflection (diagram)
[ 3 marks] [1 mark]

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(ii) State the light phenomenon involved in (a)(i).
(ii) Complete the ray diagram in Diagram 7.3
Reflection
………………………………………………… [2 marks]
[1 mark]

(iii) Based on your answer in (a)(i), state the problem
experienced by the driver in car P.

The driver in car P cannot see car Q // field of
………………………………………………………..
view very small
………………………………………………………..
[1 mark]

(b) Diagram 7.3 shows an incomplete ray diagram 1. Two reflected rays are shown (diagram)[1 mark]
when a curve mirror is placed at X to replace the 2. Angle of incidence = Angle of reflection (diagram)
plane mirror in Diagram 7.2. The curve mirror is used [1 mark]
to overcome the problem that occur in (a)(iii).
(iii) Based on your answer in b(ii), how the curve
mirror solved the problem in (a) (iii)?
The convex mirror increase the field of view
…………………………………………………….
[1 mark]

( C) The characteristics of the image formed by the
curved mirror in Figure 7.3 is diminished,
virtual and upright.

(i) What happen to the characteristics of the
image when the focal length of the curved mirror
DIAGRAM 7.3 is increased?
The driver in car P cannot see car Q // field of
(i) Give the name of the curve mirror. ………………………………………………………..
Convex mirror view very small
…………………………………[1 mark] ………………………………………………..
[1 mark]
(ii) Give the reason for your answer in (c)(i).

The characteristics of image of a convex
……………………………………………………..
mirror not depends on the focal length
…………………………………………………
[1 mark]

5-8


5.2
UNDERSTANDING REFRACTION OF LIGHT

The diagram shows the spoon bent when put inside the water.

State the Refraction of light
phenomenon
occurs.

How the Light travel from less dense medium which is air to denser medium (water), light will be
phenomenon deviated near to the normal. Thus the spoon seems like bending after putting inside the water.
occurs?

Why light is It due to change in the velocity of light as it passes from one medium into another.
refracted? Light travel more slowly in water (or glass) than in air.
When a light beam passes from air into glass, one side of the beam is slowed before the other.
This makes the beam ‘bend’.

Three different
cases of refraction

Case 1: Case 2: Case 3:

i = 0 ,r = 0 i>r i<r

5-9


When a ray of light crosses Ray is light passes from Ray of light passes from
the boundry between two air(less dense) to glass(dense) to air(less
different mediums at a right glass(denser). dense)
angle or the incident ray
parallel to normal,
 ray is bent towards  ray bent away from
 the ray is not bent the normal the normal
 but the speed of  the speed of light  the speed of light
light is different. decreases after increases after
 The angles of entering the glass emerging from the
incidence and glass.
refraction are zero.

State the Laws of The Laws Of Refraction
Refraction When the light travel from one medium to another medium which has a different optical density:

1. Snell’s Law :The ratio of the sine of the angle of incidence to the sine of the angle of
refraction is constant.

2. The incident ray, the normal and the refracted ray all lie in the same plane

Refractive Index 1. When light travels between two mediums with different optical densities, it changes speed
and bends.
2. The speed of light will decrease when it enters an optically denser medium and increases when
it enters an optically less dense medium.
3. The angle of bending of light depends on the refractive index of the mediums and the angle of
incidence ,i.

How to define 1. Refractive index, n is defined as, Example:
refractive index
n = sin i The diagram shows a ray of light passing from
sin r air to the block X.

where n = Refractive index
i = the angle in medium less
dense
r = the angle in denser medium
 A material with a higher
refractive index has a higher
density.
 The value of refractive index , n 
1 Calculate the refractive index of the block X.
 The refractive index has no units.
Solution:

n = sin 50°
sin 40°
= 1.2
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2. Example:

n = speed of light in vacuum (air) The speed of light in vacuum is 3 x 108 ms-1
speed of light in medium and the speed of light in glass is 2 x 108 ms-1 .
Determine the refractive index of glass.

Solution:
0r

n = va n = 3 x 108 ms-1
2 x 108 ms-1
vm = 1.5

3.Real Depth and Apparent Depth The refraction of light gives us a false
impression of depth.
Example:
A) The fish in the pond appears to be closer to
the surface than it actually is.

n = Real depth , H
Apparent depth, h

The following terms are defined:
Or (B) The apparent depth – a swimming pool
Real depth,H = The distance of the real object, looks shallower than it really is.
n=H
O from the surface of the water.
h
Apparent depth, h= The distance of the image,
I from the surface of the water.

5 - 11


( C) A straight object place in water
looks bent at the surface.

Explanation:

1.Rays of light from the object travel from
water to air.
2.Water is a denser medium compared to air.
3. Therefore, rays of light refract away from
the normal as they leave the water. The rays of
light then enter the eyes of the observer.
4. So the object appears to be nearer the
surface of the water.

Experiment to investigate the relationship between the Experiment to investigate the relationship between
angle of incidence and the angle of refraction. real depth and apparent depth.

Hypothesis: Hypothesis:

The angle of refraction increases as the angle of The apparent depth increases as the real depth increases.
incidence increases. .
Aim of the experiment :
Aim of the experiment :

To investigate the relationship between the angle of To investigate the relationship between apparent depth
incidence and the angle of refraction. and the real depth

Variables in the experiment: Variables in the experiment:

Manipulated variable: Angle of incidence Manipulated variable: real depth
Responding variable: Angle of refraction Responding variable: apparent depth
Fixed variable: Refractive index Fixed variable: Refractive index

List of apparatus and materials: List of apparatus and materials:

Glass block, ray box, white paper protactor, power Pin, ruler, water, retort stand ,tall beaker
supply .

5 - 12


Arrangement of the apparatus: Arrangement of the apparatus:

The procedure of the experiment which include the The procedure of the experiment which include the
method of controlling the manipulated variable and method of controlling the manipulated variable and
the method of measuring the responding variable. the method of measuring the responding variable.

The glass block is placed on a white paper. A pin is placed at the base of the beaker as object O.
The outline of the sides of the glass block are traced on The another pin is clamped horizontally onto the retort
the white paper and labelled as ABCD. stand as image position indicator, I
The glass block is removed. The beaker is filled with water.
The normal ON is drawn. By using a ruler ,the real depth of the pin is measured, H=
By using a protractor , the angle of incidence, i , is 8.0 cm
measured = 20°. The pin O is seen vertically above the surface of the
The glass block is replaced again on its outline on the water.
paper. The position of pin I is adjusted until parallax error
A ray of light from the ray box is directed along between the pin O and the pin I is non- existent.
incidence line. By using the ruler again ,the position of pin I is measured
The ray emerging from the side CD is drawn as line PQ. as the apparent depth = h
The glass block is removed again. The experiment is repeated 5 times for the other value of
The point O and P is joined and is drawn as line OP. the real depth of water, ,i.e. D=10 cm,12 cm,14 cm and
The angle of refraction, r is measured. 16 cm.
The experiment is repeated 5 times for the other angles of Tabulate the data:
incidence, i= 30° , 40°,50°, 60° and 70°.
H/cm

Tabulate the data: h/cm
Sin i
Sin r
Analysis the data:
Analysis the data: Plot the graph h against H
Plot the graph Sin r against Sin i

5 - 13


Check Yourself:
Objective Question:

1 When light travels from one medium to another, 4 Figure shows a light ray travelling from
refraction take place. Refraction is caused by the medium R to medium S.
change in the

A amplitude of light rays
B intensity of light rays
C strength of light rays
D velocity of light rays

2 An observer cannot see the coin in an empty glass Which of the following is true?
as shown in Figure(a). However , he can see the
coin when the glass is filled with water as shown A The speed of light in medium R is larger than
in Figure(b). the speed of light in medium S
B The optical density of medium R is larger
than the optical density of medium S
C The refractive index of medium R is larger
than the refractive index of medium S

5 The diagram shows a light ray directed into a
glass block.

Which is the angle of refraction?
Figure (a)
Figure (b)

The observer can see the coin in Figure (b) due to

A the total internal reflection of light
B the refraction of llight
C the reflection of light
D the diffraction of light

3 Which of the following is not caused by the
refraction of light ?
6 A light ray travels from medium P to medium Q.
A A fish in pond appears nearer to the surface
of the water Which of the following diagrams correctly shows
B The sunlight reaches to the earth in a curve the path of the light ?
path
[ Medium P denser medium and Medium Q less
C A ruler appears to bend at the water surface.
dense ]
D The sea water appear in blue colour

5 - 14


9 Which of the following formulae can be used to
determine the refractive index of a medium?

A Angle of incidence
Angle of refraction
B Apparent depth
Real depth
C Speed of light in vacuum
Speed of light in the medium

7 The diagram shows a light ray travels from liquid 10 The diagram shows a light ray travels from the air
L to liquid M. into medium X.

Which of the following diagrams correctly shows
the path of the light ? What is the refractive index of medium X?
[ Refractive index of liquid M > Refractive index A 0.85
of liquid L ] B 1.24
C 1.31
D 1.41
E 1.58

11 The diagram shows a light ray travels from the oil
into the air.

8 The diagram shows a light ray which travels from
the air to the glass.
What is the value of k?
[ Refractive index of oil = 1.4 ]

A 44.4o
B 45.6o
C 54.5o
What is the refrective index of the glass?
D 55.4o
E 58.9o
A Sin S B Sin P
Sin Q Sin R
12 The diagram shows a light of ray travels from the
air into a glass block.
C Sin Q D Sin R
Sin R Sin S

5 - 15


15 The diagram shows a coin is put at the base of
the beaker. The image of the coin appears to be 5
cm from the base of the beaker.

What is the refractive index of the glass block?

A 1.38
B 1.45
C 1.51
D 1.62 What is the refractive index of the liquid?
E 1.74 A 8 B 5
13 8
C 11 D 13
5 8
13 The speed of light in the air is 3 x 108 ms-1 .
E 19
What is The speed of light in a plastic block?
14
[ Refractive index of plastic = 1.2 ]

A 1.0 x 108 ms-1
Answer:
B 1.5 x 108 ms-1
C 2.0 x 108 ms-1 1 D 11 A
D 2.5 x 108 ms-1 2 B 12 C
E 3.0 x 108 ms-1 3 D 13 D
4 A 14 C
14 The diagram shows a boy appearing shorter when 5 D 15 D
he is in a swimming pool. The depth of the water 6 C 16
in the pool is 1.2 m. 7 B 17
[ Refractive index of water = 1.33 ] 8 C 18
9 D 19
10 A 20

What is the apparent depth of the pool?

A 0.1 m B 0.3 m
C 0.9 m D 1.1 m
E 1.6 m
5 - 16


Section A
(Paper 2)
Structure Question:

(C ) (i) Draw a Diagram of the light ray shown on
1. The Diagram shows a side view of a water-filled
diagram 3.1, meeting the water surface RS, and show
aquarium RSTU. An electric lamp, surrounded by a
its path after meeting the surface. [1 mark]
shield with a narrow transparent slit, is immersed in
one corner of the aquarium at U. The light ray from
the slit shines on the water surface RS at an angle of
40o as shown in diagram below.
R S
Water 40o 40o
R S
o
Water 40
Aquarium
Light ray
Light
ray U T
U T

DIAGRAM 3.1 ii. Calculate the angle that this new path makes with
RS and label the angle. ma
[2 [1 mark]

(a) What is meant by refractive index of a Angle = 40o
substance?
(d) The lamp is then placed outside underneath
Refractive index is an indication of the light the aquarium with the light striking to the bottom of
bending ability of the medium / the aquarium as shown in Diagram 3.2. Draw the light
ray on Diagram 3.2, after striking the aquarium.
n = sin i
sin r [1 mark] [1 mark]

(b) If the refractive index of water is 1.33,
calculate the critical angle for a ray travelling
from water to air.
water
n = 1
sin c
sin c = 1
1.33
c = 48.80 [ 2 marks]

Light ray
Lamp

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2. An observer is looking at a piece of coin at the
bottom of a glass filled with water as shown in
Diagram 3. He found that the image of the coin is 3. Figure(a) shows an object in a small pond. The
nearer to the surface of the water. depth of the water in the pond is H. The image
of the objet appears to be h from water surface.

[
2
m
a
r
k
s Figure(a)
]

(a) State the relationship between H and h
(a)(i) State a characteristic of image in When H increases, h increases/ H is directly
Diagram 3.
proportional to h
Virtual/magnified ....................................................................
[1 mark]
(b) When H = 4.5 ]m and the refractive index of
(ii) Name the science phenomenon water is 1.33, determine the value of h .
involve in the observation above.
[1 mark] n = Real depth , H
Refraction
Apparent depth, h
1.33 = 4.5 m
(b) Explain why the image of the coin H
appears nearer to the surface of the H = 3.38 m
water.
- Light ray travels from density to less
density medium (c) What happen to value of h when the pond is
- Refracted ray away from normal poured with water of refractive index 1.40 ?

[2 marks]
H decreases

(c) On Diagram 3, complete the ray ……………………………………………
diagram from the coin to the observer's
eye. [2 marks]

-Draw refracted ray correctly
- Draw ray from image to the
observer
[

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Section B (Paper 2)

Essay Question
ii. Observe Figure 4(a) and Figure 4(b) carefully.
1. Figure 4(a) shows a pencil placed in a glass of Compare the common characteristics of the pencil
water. Figure 4(b) shows the appearance of print and the print before and after they are removed
viewed from the top of a thick block of glass placed from the water and the glass block respectively.
over it. Use a physics concept to explain the appearance
of the pencil and the print in water and under the
pencil Glass block
glass block respectively.
[5 marks]

Answer:

1. The pencil appears bent when placed in water
and the print appears raised when a thick block of
glass is placed over it.
water
2. The rays of light from the pencil are refracted away
Figure 4(a) Figure 4(b) from the normal as they leave the water and enter the
eye of the observer. These rays appear to come from a
virtual image above the actual point. The pencil
(a) i. Why does the pencil appear bent to our eyes? ,therefore , appears bent in the water.
Why does the print appear raised?
[1 mark] 3. Rays of light from the print below the glass are
refracted away from the normal as they leave the glass
Answer: and enter the aye of the observer . The writing,
therefore, appears to be slightly raised.
We can see the pencil and the print because the rays
of light from the two objects reach our eyes. 4. Refraction of light is the physics concept involved.

5. Refraction of light is a phenomenon in which rays of
light change direction when they pass from one
medium to another medium of a different density.

5 - 19


5.3
UNDERSTANDING TOTAL INTERNAL REFLECTION

What is meant by Total internal reflection is the complete reflection of light ray travelling from a denser medium to
total internal a less dense medium.
reflection?
Total: because 100% of light is reflected
Internal: because it happens inside the glass or denser medium.

What is meant by The critical angle, c, is defined as the angle of incidence (in the denser medium) when the angle
critical angle ,c? of refraction (in the denser medium), r is 90°.

What are the
relationship
between the
critical angle and
total internal
reflection ?

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 When the angle of  The refracted ray  If the angle of
incidence, i keeps on travels along the incidence is
increasing, r too glass-air boundary. increased is
increases  Angle of refraction, r increased further so
= 90°. that it is greater than
 And the refracted ray
moves further away  This is the limit of the critical angle,
the light ray that can (i > c):
from the normal
be refracted in air as - no refraction
 And thus approaches the refracted in air - all the light is
the glass- air cannot be any larger totally in the
boundary. than 90°. glass
 The angle of  This phenomenon is
incidence in the called total internal
denser medium at the reflection.
limit is called the
critical angle, c.
State the two
conditions for 1. light ray enters from a denser medium towards a less dense medium.
total internal
reflection to occur 2. the angle of incidence in the denser medium is greater then the critical angle of the medium
( i > c)

What are the
relationship
between the
refractive index, n
and critical angle,
c?

What are the 1. Mirage  In hot days, a person traveling in a
phenomena car will see an imaginary pool of
involving total water appearing on the surface of
internal reflection? the road.

 The layes higher up are cooler and
denser.

 Light ray from the sky travels from
denser to less dense medium and
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will refracted away from the normal.

 The angle of incidence increases
until it reach an angle greater than
the critical angle.

 Total internal reflection occurs and
the light is reflected towards the aye
of the observer.

 If the observer’s eye is in the correct
position, he will see a pool of
water(image of the sky) appearing
on the road surface.

 This is known as a mirage.

2. Rainbow
 When sunlight shines on millions of
water droplets in the air after rain, a
multi coloured arc can be seen.

 When white light from the sun
enters the raindrops, it is refracted
and dispersed into its various colour
components inside the raindrops.

 When the dispersed light hit the
back of the raindrop, it undergoes
total internal reflection.

 It is then refracted again as it leaves
the drop.

 The colours of a rainbow run from
violet along the lower part of the
spectrum to red along the upper part.

5 - 22


Give some 1. The sparkling of a diamond  A diamond has a high refractive
examples of index.
application of total
internal reflection.  The higher the refractive index, the
smaller the critical angle.

 A small critical angle means total
internal reflection readily occurs.

 Light is easily reflected inside the
diamond.

 In this way, more light will be
confined within the diamond before
refracting out into the air.

2. Periscope  The periscope is built using two
right angled 45° made of glass. The
critical angle of the prism is 42°.

 The angle of incidence is 45° which
is greater than the critical angle.

 Total internal reflection occurs.

 The characteristics of the image are:

Virtual, upright, same size.

Give the advantages of the prism periscope
compared to mirror periscope.

Answer:

(i) The image is brighter because all the light
energy reflected.

(ii) The image is clearer because there are no
multiple images as formed in a mirror
periscope.

5 - 23


3. Prism Binocular  A light ray experiences two total
internal reflections at each prism.

 So the final image in binoculars is
virtual, upright and same size.

What are the benefits of using prism in
binoculars?

(a) an upright image is produced.

(b) The distance between the objective lens
and the eyepiece is reduced. This make the
binoculars shorter as compared to a
telescope which has the same magnifying
power.

4. Optical Fibres  The external wall of a fibre optic is
less dense than the internal wall.

 When light rays travel from a
denser internal wall to a less dense
external walls at an angle greater
than the critical angle, total internal
reflection occurs.

Give the advantage of using optical fibres
cables over copper cables.

(1) they are much thinner and lighter.

(2) a large number of signals with very little
loss over great distances.

(3) The signals are safe and free of electrical
interference

5 - 24


(4) It can carry data for computer and TV
programmes.

Check Yourself:
Objective Question:

1 A ray of red light travelling in glass strikes the [ Refractive index of medium X = 1.3
glass-air boundary . Some light is reflected and Refractive index of medium Y = 1.5 ]
some is refracted. Which diagram shows the paths
of the rays?

4 Which of the following shows total internal
reflection?

2 One of the diagram below shows the path of a
beam of
light that is incident on a water-air surface with
angle of incidence greater than the critical angle.
Which one is it?

5 The diagram shows light ray XO experiencing
total internal reflection when travelling from the
glass to air.

3 Which of the following diagram correctly shows
the total internal reflection of ray of light? Which statements about total internal reflection
are correct?
5 - 25


P -  is more than the critical angle of glass
Q - The speed of light in the glass is higher than
in air
R - The refractive index of glass is greater than
air
A P and Q
B P and R In which direction does the light move from ?
C Q and R
D P,Q and R A OQ
B OR
6 The diagram shows a semi-circular plastic block C OS
is placed in a liquid. D OT

9 A ray of light incident on one side of a
rectangular glass block. If the angle of refraction
in the glass block is 40o ,
which one of the following diagrams best
represents this ray?
[ The critical angle of glass is 42o ]
Which of the following is correct?

A Density of the plastic block is less than
density of the liquid
B Refractive index of the plastic block is
less than refractive index of the liquid
C Critical angle of the plastic block is less
than critical of the liquid
D Angle of incidence is less than critical
angle of the liquid

7 The diagram shows a ray of light passing through
medium M to medium N.

Which of the following is correct? 10 The diagram shows a light ray, P, directed into a
glass block. The critical angle of the glass is 42o.
A The angle of reflection is 55o In which direction does the light move from point
B The critical angle of medium M less than 35o Q?
C Density of medium M less than the density
of medium N

8 The figure shows a ray of light PO traveling in a
liquid strikes the liquid-air boundary.
[ The critical angle of the liquid = 45o ]
5 - 26


C the greatest angle of incidence in optically
more dense medium
D the greatest angle of incidence in optically
less dense medium

14 Which of the following shows the correct critical
angle , c of the semi- circular glass block ?

11 The diagram shows a light ray , M, directed into a
glass block. The critical angle of the glass is 42o.
In which diagram does the light move from point
O?

15 The diagram shows a light ray travelling from air
into a plastic block with an angle of incidence ,X.
What is the critical angle of the plastic?

12 The figure shows a ray of light is incident in air
to the surface of Prism A and B.

16 The diagram shows a light ray travelling from air
into a glass prism.

Which comparison is correct ?

A Density of prism A < density of prism
B
B Critical angle of prism A < critical angle of
prism B
C Refractive index of prism A < refractive
index of prism B

13 The critical angle is
What is the critical angle of the glass?
A the smallest angle of incidence in optically
more dense medium
B the smallest angle of incidence in optically
less dense medium
5 - 27


A 40o B 50o
C 60o D 70o 21 The diagram shows a cross- section of a fibre
E 80o optic cable.

17 The refractive index of water is 1.33.
What is the critical angle of the water.

A 44.5o B 46.9o
C 48.8 o D 49.2o
Which comparison is correct ?
E 54.3o
18 The refractive index of plastic block is 13 .
A Density of P < density of Q
5
B Density of P >density of Q
What is the value of the cosine of the critical
C Density of P = density of Q
angle of the plastic?

A 5 B 12
12 13

C 13 D 5
12 13

E 13
Answer:
5
1 A 11 C
19 The figure shows a ray of light AO traveling in 2 D 12 B
medium X strikes the medium X-air boundary. 3 D 13 C
[ The refractive index of medium X = 1.12 ] 4 D 14 C
5 B 15 D
6 C 16 B
7 B 17 C
8 D 18 D
9 A 19 C
10 C 20 A

In which direction does the light move from O ?

A OE B OD
C OC D OB

20 Which of the following not applies the principle
of total internal reflection?

A Prism binocular
B Mirror periscope
C Optical fibre
D Road mirage
5 - 28


Section A (c) Name other optical device that applies the
(Paper 2) phenomenon in (a)(i).
Structure Question: [ 1 mark ]

Prism periscope // prism binoculars // camera//
1. Diagram 1 shows a cross-sectional area of an
endoscope and etc.
optical fibre which consist of two layers of glass
with different refractive index. The glass which 2. Figure 4 shows a traveller driving a car on a hot
forms the inner core, Y is surrounded by another day. The traveller sees a
type of glass which forms the outer layer, X. puddle of water on the road a short distance ahead
of him.

Puddle of water

Figure 4

DIAGRAM 1
(a) (i) Name the light phenomenon observed in a) Which part of the air is denser?
optical fibre?
Close to the sky / cool air
Total internal reflection
…………………………………………………………
( 1 mark )

[ 1 mark ] b) Name a phenomena of light that always depends
(ii) Compare the refractive index of outer on the air density when light travels from the sky to
layer X and inner core Y. the earth before it reaches point X.
The refractive index of Y is higher than
the refractive index of X// Vice versa Refraction
…………………………………………………………
[ 1 mark ] ( 1 mark )

c) i) What is the phenomenon occurring at point X
(b) The refractive index of inner core Y is 2.10.
Calculate the critical angle of the inner core Total internal reflection
Y. ……………………………………………………..…
(1 mark )
1 1
Sin c = = = 0.4762
n 2 .1 ii) What is the puddle of water actually?

c = 28.44o // 280 26’ The image of sky
[2 marks] …………………………………………………………
( 1 mark )
5 - 29


d) Using the diagram above, explain how the traveller
can see the puddle of
water on the road.

1.Light from sky to the earth refracted

2. The light reach at a point X, total internal
reflection occurred

…………………………………………………………
( 2 marks )

e) Name one optical instrument that uses the
phenomenon in (d)

Optical fibre
………………………………………………………… Answer:
( 1 mark )
Glass prism
Object
3. Completing the ray diagram below, to show how a 45o
periscope works: (critical angle of glass = 42o) Total internal
reflection takes
place because
angle of incident >
critical angle
Glass
object
prism
tctct

Eye

Eye
e

5 - 30


5.4
U N D E R S T A N D I N G L E N S E S

Introduction
Lenses are made of transparent material such as glass or clear plastics.

They have two faces, of which at least one is curved.

Types of lenses (a) Convex lens, also known as converging (b) Concave lens, also known as diverging
lens. lens.
It is thicker at the centre of the lens. It is thinner at the centre of the lens.

Convex lens Concave lens
State the
differences
between convex
lens and concave
lens

When light ray which are parallel and close to When light rays are parallel to the principle
the principle axis strikes on a convex lens, they axis fall on a concave lens., they are refracted
are refracted and converge to a point, F on the and appear to diverge from the focal point on
principle axis. This point is a focal point of the the principle axis.
convex lens.

Common 1. The focal point, F is a point on the principle axis where all rays are close and parallel to the
terminology of axis that converge to it after passing through a convex lens, or appear to diverge from it after
reflection of light passing through a concave lens.
on a curved mirror
2. The focal length, f is the distance between the focal point and the optical centre.

3. The optical centre, C is the geometric centre of the lens. It is the point through which light
rays pass through without deviation.

4. The principle axis is the line passing through the optical centre, C.

5 - 31


Construction rules Rule 1:
of convex lens A ray parallel to the principle axis is
refracted through the focal point, f.

Rule 2:
A ray passing through the focal point is
refracted parallel to the principle axis.

Rule 3:
A ray passing through the optical
centre, C travels straight without
bending.

The point of intersection is the position
of the image.
The images formed by a convex lens
depend on the object distance, u.

Images form by Using the principles of construction of ray diagram, complete the ray diagrams for each of the
convex lens cases shown below:

u = object distance; v = image distance ; f = focal length

Note: Point of intersection in the position of the image

image: 1.magnifying glass
1.virtual spectacle
2.upright 2.lens for long-
3.magnified sightedness.
4.Same side as the
object

B u = f ( Object, O is at F ) Characteristics of Application:
image: 1. to produce a
1.virtual parallel a parallel
2.upright beam of light , as in a
3.magnified spotlight,
4. Same side as the astronomical
object telescope

5 - 32


C f < u < 2f or f < u < r ( Object O is Characteristics of Application:
between F and C image: 1.projector lens
1.real 2.photograph
2.inverted 3. enlarger
3.magnified 4.objective lens of
4. On apposite side microscope
of the object.

D u = 2f or u = r ( Object ,O is at C) Characteristics of Application:
image: 1.photocopying
1.real machine
2.inverted
3.same size as the
object
4. On the opposite
side of the object

Eu > 2f or u > r ( Object, O is beyond C ) Characteristics of Application:
1.real 2.sharing mirror
2.inverted 3. make-up mirror
3.magnified

F u =  ( Object ,O very far from the lens) Characteristics of Application:
1.virtual 2.sharing mirror
2.upright 3. make-up mirror
3.magnified

5 - 33


Construction rules
of concave lens Ray 1:
A ray parallel to the principle axis is refracted
1
as if it appears coming from the focal point, F
which is located at the same side of the
2 incident ray.

3 Ray 2:
A ray passing through the focal point is
refracted parallel to the principle axis.

Ray 3:
A ray passing through the optical centre, C
travels straight without bending.

The point of intersection is the position of the
image .
The image formed by a concave lens are
always :

Virtual, upright and diminished.

Image formed by Using the principles of construction of ray diagram, complete the ray diagrams for each of the
convex mirror: cases shown below:

u = object distance; v = image distance ; f = focal length

1.diminished mirror
3.upright mirror

(B ) f<u <2f (object at between f and 2f) Characteristics of Application:
1.diminished mirror
3.upright mirror

5 - 34


Power of lenses Power of a lens = 1 .
Focal length The unit of power of a lens is
Dioptre (D) or m-1
or P = 1 @ P = 100 Convex lens : the power is taken to be
f f( cm) positive
Concave lens : the power is taken to be
negative

f = focal length

Linear Magnification (m) :

The linear
magnification , m
define as:
Where

f = focal length
u = object distance
v = image distance
m = linear magnification
Lens Formula
ho = object height
hi = image height

Sign Conventions Type of lenses Convex lens Concave lens
Object distance ,u Always + Always +
Object is always placed to the Object is always placed to the
left of the lens left of the lens
Image distance, v + if the image is real ( image + if the image is real ( image
is formed on the right side of is formed on the right side of
the lens. the lens
- if the image is virtual - if the image is virtual
( image is formed on the left ( image is formed on the left
side of the lens). side of the lens).

Focal length, f Always + Always -
Power of length, P Always + Always -
Linear magnification, m Size of image
ImI =1 Image and object are the same size

ImI >1 Enlarged image

ImI <1 Diminished image

5 - 35


Meaning of real A real imge is one which can be cast on a A virtual imge is one which cannot be cast on a
image and virtual screen. screen.
image

Check Yourself:
Objective Question:

1 The image produced by a lens is caused by the
A total internal reflaction of ray
B diffraction of ray
C refraction of ray
D reflection of ray

2 The diagram shows parallel rays of light is
incident to a combination of plastics with
different refractive index.

4 Which of the following drawing is not correct
path of the light rays?

Which of the following diagrams is correct?

5 Which of the following is true?

A The unit of the power of lens is Watt
3 The diagram shows parallel rays of light passing B The power of a convex lens is negative
through a liquid in glass container. C A lens with a shorter focal length has a
[ The refractive index of the liquid = 1.35 ] lower power
D The rays of light passes through the optical
centre of lens without any refraction

Which of the following diagrams is true?

5 - 36

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