5.0 light

Physics Module Form 4 Chapter 5 - Light GCKL 2011

5.1
UNDERSTANDING REFLECTION OF LIGHT

What light is? Is a form ……………Light travel in a .................. 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 ……………….ray.
by the surface 2. The light ray that bounces off from the
of mirror? surface of the mirror is called
……………..
3. The ……………is a line perpendicular
to the mirror surface where the reflection
occurs.
4. The angle between the incident ray and
AO = incident ray
the normal is called the ………………
OB = reflected ray
5. The angle between the reflected ray and
i = angle of incident
the normal is called
r = angle of reflected
…………………………………………….

What is the The Laws Of Reflection
Law of 1. The incident ray, the reflected ray and the normal …………………………………..The
Reflection ? angle of incident, i, is ………………..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.

5-1


State the 1.
characteristics 2.
of the image 3.
formed by 4.
plane mirror 5.

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

What is meant
by real image?
Image that ………………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 = …………………………… 3. PF= ………………………………
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 = ……………………………………………………………………………….
light on a ...........................................................................................................
curved mirror 2.Pole of mirror, P = ……………………………………………………………………….
3.Radius of curvature ,r = CP = ……………………………………………………………
4.Focal length, f = ………………………………………………………………………….
................................................................................................................
5.Object distance, = ………………………………………………………………………..
6.Object distance , v = ..........................................................................................................
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.
1. 2.
2. 3.
3.

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

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

I

D u = 2f or u = r ( Object ,O is at C) Characteristics of Application:
image: 1.
1.
2.
3.

5-3


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

I

F u =  ( Object ,O very far from the lens) Characteristics of Application:
image:
1.
2.
3.

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.
1. 2.
2.
3.

5-4


Check Yourself:
Objective Question:

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

2. The diagram shows a single ray of light being 5. An object is placed in front of a plane mirror.
directed at a plane mirror. Compare to the object, the image formed in the
mirror is always

A virtual
40° B smaller
C bigger
D three times as far away
What are the angles of incidence and reflection?
Angle of incidence Angle of reflection
A 40o
40o 6. A light ray incident onto a plane mirror at an
B 40o
50o angle of 50o
C 50o 40o The characteristics of an image , formed by a
D 50o 50o convex mirror for all positions of the object are
A diminished, real and inverted
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

5-5


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
C virtual, inverted and magnified DIAGRAM 3.1 / RAJAH 3.1
D virtual, upright and diminished

9. When an object is placed at a point 20 cm in (a) Name the type of mirror shown in Diagram 3.1
front of a concave mirror, a real image of the
same as the object is formed on a screen placed ……………………………………………………..
next to the object. What is the focal length of the [1 mark]
mirror?
(b) Name one characteristic of the image formed
A 5 cm by the mirror.
B 10 cm
C 15 cm ……………………………………………………..
D 20 cm [1 mark]
(c) Sketch a ray diagram to show how the image
10. Which of the following states the right reason for
is formed.
replacing a plane mirror are used as rear- view
mirrors in motor vehicles with a convex mirror ?

A To shine the object
B To widen the field of view
C To produce a brighter image
D To produce a sharper image

Answer:

1
2
3
4
5 [3 marks]
6 (d) What is the advantage of using this type of
7 mirror in the shop?
8
9 ……………………………………………………………
10 [1 mark]
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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.

………………………………………… …….
[ 1 mark ]

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

(c) State two characteristics of the image
formed.

………………………………………………
[ 2 marks ]

(d) In the diagram below, the arrow represents
the teeth as the object of the mirror. DIAGRAM 7.2
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]

[ 3 marks]
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(ii) State the light phenomenon involved in (a)(i). (ii) Complete the ray diagram in Diagram 7.3

[2 marks]
…………………………………………………
[1 mark]

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

………………………………………………………..

..............................................................................
[1 mark]

(b) Diagram 7.3 shows an incomplete ray diagram
when a curve mirror is placed at X to replace the
plane mirror in Diagram 7.2. The curve mirror is used
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)?

…………………………………………………
[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
DIAGRAM 7.3 image when the focal length of the curved mirror
is increased?
(i) Give the name of the curve mirror.
………………………………………………………..
…………………………………[1 mark]
………………………………………………..
[1 mark]
(ii) Give the reason for your answer in (c)(i).

……………………………………………………..

…………………………………………………
[1 mark]

5-8


5.2
UNDERSTANDING REFRACTION OF LIGHT

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

State the
phenomenon ………………………………….
occurs.

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

Why light is It due to change in the …………………of light as it passes from one medium into another.
refracted? Light travel more ………………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 ………………………….

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
………………..to normal,
 ray is bent  ray bent
 the ray is ……………………... ………………..from
…………………but the normal the normal
the speed of light is
…………………….  the speed of light  the speed of light
The angles of …………………after …………………….
incidence and entering the glass after emerging from
refraction the glass.
are………………….

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……………………………………………………………………………………………………

2. ………………………………………………………………………………………………….

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 looks shallower than it really is.
n=H
……………… O from the surface of the water.
h
Apparent depth, h= The distance of the
………………..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
………………………
2.Water is a ……………………..medium
compared to air.
3. Therefore, rays of light
………………….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
…………………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 ………………. as the angle of The apparent depth …………………..as the real depth
incidence ……………………… ………………………..
.
Aim of the experiment : Aim of the experiment :
To investigate the relationship between
…………………………. and …………………….. To investigate the relationship between
……………….and the …………………………………..
Variables in the experiment: Variables in the experiment:

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

5-13


Check Yourself:
Objective Question:

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

Which of the following is true?
2 An observer cannot see the coin in an empty glass
as shown in Figure(a). However , he can see the A The speed of light in medium R is larger than
coin when the glass is filled with water as shown the speed of light in medium S
in Figure(b). 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 Which of the following diagrams correctly shows
of the water the path of the light ?
B The sunlight reaches to the earth in a curve
[ Medium P denser medium and Medium Q less
path
dense ]
C A ruler appears to bend at the water surface.
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
L to liquid M.

10 The diagram shows a light ray travels from the air
into medium X.

Which of the following diagrams correctly shows
the path of the light ?
[ Refractive index of liquid M > Refractive index
of liquid L ]
What is the refractive index of medium X?
A 0.85
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
What is the refrective index of the glass?
C 54.5o
A Sin S B Sin P D 55.4o
Sin Q Sin R E 58.9o

C Sin Q D Sin R
Sin R Sin S

5-15


12 The diagram shows a light of ray travels from the 15 The diagram shows a coin is put at the base of
air into a glass block. 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 What is the refractive index of the liquid?
B 1.45 A 8 B 5
C 1.51 13 8
D 1.62 C 11 D 13
E 1.74 5 8
E 19
14
13 The speed of light in the air is 3 x 108 ms-1 .
What is The speed of light in a plastic block?
[ Refractive index of plastic = 1.2 ]

A 1.0 x 108 ms-1
B 1.5 x 108 ms-1 Answer:
C 2.0 x 108 ms-1
D 2.5 x 108 ms-1 1 11
E 3.0 x 108 ms-1 2 12
3 13
14 The diagram shows a boy appearing shorter when 4 14
he is in a swimming pool. The depth of the water 5 15
in the pool is 1.2 m. 6 16
[ Refractive index of water = 1.33 ] 7 17
8 18
9 19
10 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
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
substance?
(d) The lamp is then placed outside underneath
[1 mark] the aquarium with the light striking to the bottom of
the aquarium as shown in Diagram 3.2. Draw the light
ray on Diagram 3.2, after striking the aquarium.
(b) If the refractive index of water is 1.33,
calculate the critical angle for a ray travelling [1 mark]
from water to air.

[ 2 marks]

water

Light ray
Lamp

5-17


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

[
2
m
a
r
Figure(a)
k
s
]
(a) State the relationship between H and h

....................................................................
(a)(i) State a characteristic of image in
Diagram 3. (1 mark)

(b) When H = 4.5 m and the refractive index of
[1 mark]
water is 1.33,[ determine the value of h .

1 (2 marks)
1
]
(ii) Name the science phenomenon
involve in the observation above.
[1 mark]

(b) Explain why the image of the coin
appears nearer to the surface of the (c) What happen to value of h when the pond is
water. poured with water of refractive index 1.40 ?
[2 marks]
……………………………………………
(c) On Diagram 3, complete the ray
diagram from the coin to the observer's (1 mark)
eye. [2 marks]

[

1
]

5-18


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
pencil Glass block of the pencil and the print in water and under the
glass block respectively.
[5 marks]

water

Figure 4(a) Figure 4(b)

(a) i. Why does the pencil appear bent to our eyes?
Why does the print appear raised?
[1 mark]

Answer:

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 …………………
total internal to a .................................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 ?

5-20


 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.
total internal
reflection to occur 2.

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
will refracted away from the normal.
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 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.

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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:

1.
………………………………………………
………………………………………………
………………………………………………
…………………………………………..

2.

………………………………………………
………………………………………………
………………………………………………
……………………………………………

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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?

1.

………………………………………………
………………………………………………
………………………………………………

2.

………………………………………………
………………………………………………
………………………………………………

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. .................................................................

2. .................................................................

.................................................................

3. .................................................................

.................................................................

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4. .................................................................
.................................................................

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.

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

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 O ?
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
12 The figure shows a ray of light is incident in air into a plastic block with an angle of incidence ,X.
to the surface of Prism A and B. What is the critical angle of the plastic?

Which comparison is correct ?
16 The diagram shows a light ray travelling from air
A Density of prism A < density of prism B into a glass prism.
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

A the smallest angle of incidence in optically
more dense medium
B the smallest angle of incidence in optically
less dense medium
What is the critical angle of the glass?

5-27


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

17 The refractive index of water is 1.33.
What is the critical angle of the water.
A 44.5o B 46.9o
o Which comparison is correct ?
C 48.8 D 49.2o
E 54.3 o
A Density of P < density of Q
B Density of P >density of Q
18 The refractive index of plastic block is 13 .
C Density of P = density of Q
5
What is the value of the cosine of the critical
angle of the plastic?

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

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 ]
1. Diagram 1 shows a cross-sectional area of an
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
optical fibre?
a) Which part of the air is denser?

…………………………………………………………
[ 1 mark ] ( 1 mark )
(ii) Compare the refractive index of outer
layer X and inner core Y. b) Name a phenomena of light that always depends
on the air density when light travels from the sky to
the earth before it reaches point X.
[ 1 mark ]
…………………………………………………………
( 1 mark )
(b) The refractive index of inner core Y is 2.10.
Calculate the critical angle of the inner core
Y. c) i) What is the phenomenon occurring at point X

……………………………………………………..…
[ 2 marks ] (1 mark )

ii) What is the puddle of water actually?

…………………………………………………………
( 1 mark )
5-29


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

…………………………………………………………

…………………………………………………………
( 2 marks )

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

…………………………………………………………
( 1 mark )

3. Completing the ray diagram below, to show how a
periscope works: (critical angle of glass = 42o)

Glass
object
prism
tctct

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 …………………..material such as glass or clear plastics.

They have two faces, of which at least one is ……………………

Types of lenses (a) …………………lens, also known as (b) ………………lens, also known as
converging lens. diverging lens.
It …………………….at the centre of the lens. It is ………………….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 …….. on a convex lens, they axis …… on a concave lens., they are ………..
are ………… and ............... to a point, F on the and appear to …………. from the ……………
principle axis. This point is a ………….. of the on the principle axis.
convex lens.
Common 1. The focal point, F is a point on the …………………….where all rays are ……………….and
terminology of ……………………..to the axis that ………………………to it after passing through a convex
reflection of light lens, or appear to ………………………..from it after passing through a concave lens.
on a curved mirror
2. The focal length, f is the distance between the …………………and the ………………….

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

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 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.
1. 2.
2.
3.
4.

B u = f ( Object, O is at F ) Characteristics of Application:
image: 1.
1.
2.
3.
4.

5-32


C f < u < 2f or f < u < r ( Object O is Characteristics of Application:
between F and C image: 1.
1. 2.
2. 3.
3. 4.
4.

D u = 2f or u = r ( Object ,O is at C) Characteristics of Application:
image: 1.
1.
2.
3.
4.

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

F u =  ( Object ,O very far from the lens) Characteristics of Application:
image: 1.
1. 2.
2. 3.
3.

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

image: 1.
1. 2.
2.
3.

(B ) f<u <2f (Object at between f and 2f) Characteristics of Application:
image: 1.
1. 2.
2.
3.

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?
3 The diagram shows parallel rays of light passing
through a liquid in glass container. A The unit of the power of lens is Watt
[ The refractive index of the liquid = 1.35 ] B The power of a convex lens is negative
C A lens with a shorter focal length has a
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


6 Diagram shows light rays passing through a 10 Figure shows four light rays A,B,C and D passing
convex lens . through a convex lens. F is the focal point of the
length. Which of the following path of the light
rays is not correct?

What is the distance P ?

A Image distance
B Object distance
C Focal length 11 What is the power of a convex lens which has a
D Optical length focal length 50 cm ?

7 The diagram shows the action of a magnifying A -0.2 D B 0.2 D
glass. C -2.0 D D 2.0 D
Which point is the principal focus of the lens? E 4.0 D

12 The power of a lens is - 40 D. What is the type
of the lens and its focal length?

Type of length Focal length
2
A Concave lens -2.5 x 10 m
-2
B Concave lens -2.5 x 10 m
8 The diagram shows an image ,I is formed by a 2
C Convex lens -2.5 x 10 m
convex lens.
D Convex lens -2.5 x 10-2 m
Where is the position of the object?
E Concave lens -4.0 x 10-2 m

13 A convex lens of focal length ,f. The lens
produces a enlarged , virtual and upright
image.The object distance is

A less than f
9 The diagram shows an image ,I is formed by a B between f and 2f
concave lens. C same as 2f
Where is the position of the object?
D more than 2f

14 A light bulb is placed at the principal focus of a
convex lens. After travelling through the lens the
rays of light is

A parallel
B converge
C diverge

5-37

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