Chapter 5

Light

PHYSICSFORM 4

Cikgu DesikanCompiled by

1. Understanding reflection of light

2. Understanding refraction of light

3. Understanding total internal reflection of light

4. Understanding lenses

Chapter 5

LightP

RE

SP

M P

HY

SIC

S

2016

Learning Objectives :

Dear students,

The two basic processes of education are knowing and valuing.

2007 2008 2009 2010 2011 2012 2013 2014 2015

P1 5 5 5 4 5 5 5 4

P2

A 1 1 1 1 1 1 1 1

B - - - - - - - -

C - 1 - - - - - -

P3A - - - - - - - -

B 1 - - - 1 - 1 -

Analysis of Past Year Questions

Chapter 5

Light

Concept Map

Dear students,

Light

LensesReflection of

Light

Refraction of

Light

Laws of

Reflection

Laws of

Refraction

Refractive

index

Convex

Lens

Ray

diagram

Concave

lens

Mirror

Ray Diagram

Positions and

characteristics of

image

v

1

u

1

f

1

Positions and

characteristics of image

Total internal

Reflection

sinc

1n

r sin

i sinn

medium in light of v

vacuum in light of vn

d

Dn

Plane

Convex

Concave

Great dreams of great dreamers are always transcended.(by Dr. Abdul Kalam)

5.1 Understanding Reflection Of Light

Reflection

1. Mirror works because it reflects light..

2. The light ray that strikes the surface of the

mirror is called incident ray.

3. The light ray that bounces off from the

surface of the mirror is called reflected ray.

4. The normal is a line perpendicular to the

mirror surface where the reflection occurs.

5. The angle between the incident ray and the

normal is called the angle of incidence, i

6. The angle between the reflected ray and the

normal is called the angle of reflection, r.

Laws of Reflection

NBA

Mirror O

AO = Incident ray

OB = Reflected ray

i = Angle of incident

r = Angle of reflection

i r

4

Draw ray diagrams to show the positioning and characteristics of the image formed by a

plane mirror.

Characteristics of the image formed by

reflection of light.

Notes:

Real image : Image that can be seen on a

screen

Virtual image : Image that cannot be seen

on a screen.

Plane Mirror Student

Object

5

Great dreams of great dreamers are always

transcended. “

” APJ Abdul Kalam

Concave Mirror

Reflection of light on curved mirror

Convex Mirror

Common terminology of curved mirrors

Centre of curvature, C

The center of sphere of the mirror

Principle axis

The connecting line from the centre of curvature to point P

Radius of curvature, CP

The distance between the centre of curvature and the surface

of the mirror.

Focal point, F

The focal point of a concave mirror is the point on the principle

axis where all the reflected rays meet and converge.

The focal point of convex mirror is the point on the principle

axis where all the reflected rays appear to diverge from

behind the mirror.

6

C FP

CFP

Differences

Common terminology of curved mirrors

Focal length, f

The distance between the focal point and the surface of the mirror. (FP or ½ CP)

Object distance, u

The distance between the object and the surface of the mirror.

Image distance, v

The distance between the image and the surface of the mirror.

Concave Mirror Convex Mirror

Rays travelling parallel to the principal axis

converge to a point, called the focal point on

the principal axis.

Rays travelling parallel to the principal axis

appear to diverge from a point behind the

mirror, called the focal point on the principal

axis.

7

Concave Mirror Convex Mirror

Rule 1

A ray parallel to the principal axis is reflected

through F.

A ray parallel to the principal axis is reflected as

if it comes from F.

Rule 2

A ray passing through F is reflected parallel to

the principal axis

A ray directed towards F is reflected parallel to

the principal axis.

Construction Rules for Concave Mirror and Convex Mirror

Object

F

Object

F

Object

F

Object

F

8

Construction Rules for Concave Mirror and Convex Mirror

Concave Mirror Convex Mirror

Rule 3

A ray passing through C is reflected back along

the same path through C.

A ray is directed towards C is reflected back

along the same path away from C.

If an egg is broken by an outside force….

A life ends.

If an egg breaks from within...... .

Life begins.

Great things always begin from within .

“

”

Object

F C

Object

F C

9

O

C F

u > 2f u = 2f or u = c

f < u < 2f or f < u < c u = f

O C F

O

F

O

C F

10

Ray Diagrams of concave mirror

u < f Object

distance

Characteristics of the

image:

u > 2f

u = 2f

f < u < 2f

u = f

u < f

Ray Diagram of concave mirror

Excellent ! We can attack in

any direction.

O

C F

11

Sir, we are surrounded from all

sides by enemies!

f < u < 2f

Ray Diagram of convex mirror

u < f

O

C F F C

O

C F F C

Object distance Characteristics of the image:

u > 2f

• Diminished, upright, virtual

• Image formed within 0 < v < f

u = 2f

f < u < 2f

u = f

u < f

12

Application of Reflection of Light

13

Anti-parallax Mirror in Ammeters or Voltmeter

1. A parallax error occurs when the scale is viewed at

an improper angle (the eye sees both the pointer and

its image).

2. Some meters provide a mirror within the display, so

that a user can easily determine the correct viewing

angle by checking the needle's reflection.

3. The proper angle is achieved when the needle's

reflection is not visible to the user's eye.

Periscope

mirror

strip

pointer

pointer’s image

1. A periscope can be used to see over the top

of high obstacles such as a wall.

2. It is also used inside a submarine to observe

the surrounding above water surface.

3. Consist of 2 plane mirror inclined at an

angle of 45°.

4. The final image appears upright.

ray from a

far object

45°

45°

mirror

14

Ambulance

• Why is the word ‘AMBULANCE’

purposely inverted laterally on

an ambulance car?

• Images seen through the rear

mirror of a car is laterally

inverted.

Make-up Mirror

Concave mirrors with

long focal lengths,

produce virtual,

magnified and upright

images

• The light bulb is fixed in position at the focal

point of the concave mirror to produce a

beam of parallel light rays.

• The beam of parallel light rays will maintain

a uniform intensity for a greater distance.

• Other applications are the headlight of motor

vehicles and the lamp of slide projectors.

Reflector of torchlight

parallel

light rays

bulb

F

Field of vision

a) Plane mirror

Wider field of vision

b) Convex mirror

15

Widening the field of vision

• When a convex mirror is used, the field of vision is larger than a plane mirror

• Convex mirrors are used as rear view mirrors in motor vehicles to give drivers a wide-angle view

of vehicles behind them.

• It is also used as shop security mirrors.

A concave parabolic surface is

used to focus the radio wave

signals.

Transmission of radio waves and signals

5.2 Understanding Refraction Of Light

Refraction of light

Angle of

incidence, i

the angle between the incident ray

and the normal.

Angle of

refraction, r

the angle between the refracted ray

and the normal

i > r the ray bent towards the normal, and

the speed of light decreases.

r < i the ray bent away from the normal

and the speed of light increases.

NADenser

medium

O

i

r

Less Dense medium

NALess Dense

medium

O

i

r

Denser medium

B

B

AO = Incident ray

OB = Refracted ray

ON = Normal line

i = Angle of incident

r = Angle of refraction 16

When a light ray travels from

less dense medium to denser

medium

When a ray of light travels

from denser medium to less

dense medium.

When light ray is incident

normally on the boundary

between the two medium.

The light ray is refracted

towards the normal.

The speed of light decreases.

The light ray is refracted away

from the normal.

The speed of light increases.

The light ray is does not bend.

3 ways in which a ray of light can travel through two medium

The Laws of Refraction

17

NADenser

medium

O

i

r

Less Dense mediumB

NALess Dense

medium

O

i

r

Denser mediumB

nconstantr sin

i sin

Snell’s Law

• The refraction of light is caused by the

change in velocity of light when it

passes from a medium to another

medium.

• The refractive index has no units.

• It is an indication of the light-bending

ability of the medium as the ray of light

enters its surface from the air.

Refractive Index, n

1

medim in light of velocity

vacuum in light of velocityn

2

Real Depth and Apparent Depth

1. Rays of light coming from the real fish, O

travels from water (more dense) to air

(less dense)

2. The rays are refracted away from the

normal as they leave the water.

3. When the light reaches the eye of the

person, it appears to come from a virtual

fish, I which is above the real fish O.

h

Hn 3

18

Velocity of light in medium

h = Aparent depth

H = Real depth

Air

O

H

hWater

I

Normal

19

a) Draw a ray diagram from point P to the eye to show how the legs appear shorter.

b) The depth of water is 0.4 m. Calculate the distance of the image of the foot at point P from

the surface of the water. [Refractive index of water = 1.33]

Exercise 5.2

1.

3. A light ray is incident normally on a glass prism which has a

refractive index of 1.50.

a) complete the ray diagram.

b) Find the incident angle and the refractive angle

20

2. The light ray travels from air to medium x. Find the:

a) incident angle

b) refracted angle

c) refractive index 60° Medium X

45°

Air

30°

60°

5.3 Total Internal Reflection Of Light

1. When light travels from a denser medium to a less

dense, it bends away from normal.

2. A small part of the incident ray is reflected inside the

glass.

3. The angle of refraction is larger than the angle of

incidence, r > i

i < c

1. When the angle of incidence, i keeps on increasing, r

too increases and the refracted ray moves further

away from the normal – and thus approaches the

glass – air boundary.

2. The refracted ray travels along the glass-air boundary.

3. This is the limit of the light ray that can be refracted in

air as the angle in air cannot be any larger than 90°.

4. The angle of incidence in the denser medium at this

limit is called the critical angle, c.

i = c

21

c

Glass

Air

Normal

Incident

ray Weak

reflected

ray

r = 90°Refracted

ray

i = c c

i

Glass

Air

Normal

Incident

ray Weak

reflected

ray

Refracted

rayr

1. If the angle of incidence is increased further so that it

is greater than the critical angle, the light is not

refracted anymore, but is internally reflected.

2. This phenomenon is called total internal reflection.

i > c

The two conditions for total internal reflection to occur are:

1. light ray enters from a denser medium towards a less dense medium

2. the angle of incidence in the denser medium is greater than the critical angle of the medium.

Total internal reflection

Conditions

22

Glass

Air

Normal

Incident

ray

Strong

reflected

ray

i > cc

Figure shows a light ray strikes the surface of a prism. The

refractive index of glass is 1.5. Find the critical angle. Complete

the path of the light ray that passes into and out of the prism.

Calculate the critical angle, c [ Refractive index of water = 1.33 ].

Exercise 5.3

23

45°

c

Water

Air

1.

2.

Natural Phenomenon involving Total Internal Reflection

1. Mirage is caused by refraction

and total internal reflection.

2. Mirage normally occur in the

daytime when the weather is hot.

3. The air above the road surface

consists of many layers.

4. The layers of air nearest the road are hot and the layers get cooler and denser towards the

upper layers.

5. The refractive index of air depends on its density. The lower or hotter layers have a lower

refractive index than the layers above them.

Mirage

Sunset 1. The Sun is visible above the horizon

even though it has set below the

horizon.

2. Light entering the atmosphere is

refracted by layers of air of different

densities producing an apparent shift

in the position of the Sun.

24

Applications of Total Internal Reflection

1. The periscope is built using two right-angled

prisms.

2. The critical angle of the glass prisms is 42°.

3. Total internal reflection occurs when the light rays

strike the inside face of a 45°angles with an angle

of incidence, I, greater than the critical angle, c,.

4. The image produced is upright and has the same

size as the object.

Advantage of the prisms periscope compared to a

mirror periscope:

a) the image is brighter because all the light energy

is reflected.

b) the image is clearer because there are no multiple

images as formed in a mirror periscope.

Prism Periscope

25

object

image

prism

prism

45°

45°

refraction

Total internal reflectionFish sees outside

world inside 96˚

cone

1. A fish is able to see an object above the water surface because the rays of light from the object

are refracted to the eyes of the fish or diver.

2. Due to total internal reflection, part of the water surface acts as a perfect mirror, which allows

the fish and diver to see objects in the water and the objects around obstacles.

3. A fish sees the outside world inside a 96° cone. Outside the 96°cone, total internal reflection

occurs and the fish sees light reflected from the bottom of the pond. The water surface looks

like a mirror reflecting light below the surface.

Fish’s Eye View

26

1. A pair of binoculars uses two prisms which

are arranged as shown in figure.

2. Light rays will be totally reflected internally

two times in a pair of binoculars.

3. The benefits of using prisms 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.

Prism Binoculars

27

Prism A

Prism BObjective

lens

Eyepiece

lens

Object

Image

45°

45°

1. Fiber optics consists of a tubular rod

which is made from glass and other

transparent material.

2. The external wall of a fiber optic is less

dense than the internal wall.

3. When light rays travel from a denser

internal wall to a less dense external wall

at an angle that exceeds the critical angle,

total internal reflection occurs repeatedly.

4. This will continue until the light rays enter

the observer’s eye.

5. Optical fiber is widely used in

telecommunication cables to transmit

signal through laser. It can transmit signal

faster and through long distance with high

fidelity.

6. Optical fiber is also used in an endoscope

for medical emerging.

Advantage of using optical fibres cables over copper cables:

(a) much thinner and lighter

(b) a large number of signals can be sent through them at one time.

(c) transmit signals with very little loss over great distances.

(d) signals are safe and free of electrical interference

(e) can carry data for computer and TV programmes.

Optical fibers

28

Internal wall

external wall

5.4 Lenses

Lenses are made of transparent material such as glass or clear plastics. They have two faces, of

which at least one is curved.

Convex Lens

Concave Lens

Convex lenses @

converging lenses

- thicker at the centre

Concave lenses @

diverging lenses

- thinner at the centre

29

Biconvex Plano-convex Concavo - convex

Biconcave Plano-concave Concavo - concave

Focal Point and Focal Length of a Lens

A point on the principle axis to which incident

rays of light traveling parallel to the axis

converge after refraction through a convex

lens.

Focal Point @ the principal focus, F

Distance between the focal point , F and

optical centre, C on the lens.

30

Focal length, f

Distance between the focal point, F and the

optical centre , C

Focal Point @ principal focus, F

A point on the principal axis to which incident

rays of light traveling parallel to the axis

appear to diverge after refraction through a

concave lens.

Focal length, f

CF F

Light rays

Principal axis

Focal

point

Optical center

CFPrincipal axis

Light rays

Focal

point

f

f

The ray parallel to the

principal axis is refracted

through the focus point, F.

A ray passing through the

focus point is refracted

parallel to the principal axis.

A ray passing through the

optical centre travels straight

without bending.

The ray parallel to the

principal axis is refracted as if

it appears coming from focus

point, F which is located at

the same side of the incident

ray.

A ray passing the focus point

is refracted parallel to the

principle axis.

A ray passing through the

optical centre travels straight

on without bending.

Concave Lens

Rules for Ray Diagrams

Convex Lens

31

OF FOF F

O

FF

OF F OF FO FF

1 2 3

1 2 3

Ray Diagrams of convex lens

OF2F F

u < f

OF2F F

u = f

OF2F F

f < u < 2f

2F

32

OF

F

u =

OF2F F

u = 2f

2FO

F2F F

u > 2f

Object

distance

Characteristics of the image:

u = Diminished, inverted, real

u > 2f Diminished, inverted, real

u = 2f Same size, inverted, real

f < u < 2f Magnified, inverted, real

u = f Magnified, upright, virtual

u < f Magnified, upright, virtual

33

Ray Diagrams of concave lens

OF2F

f < u < 2f

Object

Image

R1

R3

34

Ray Diagrams of concave lens

OF2F

u = 2f

Object

distance Characteristics of the image:

u =

u > 2f

u = 2f

f < u < 2f

u = f

u < f

OF2F

u < f

35

Power of Lenses

1. The power of a lens is a measure of its ability to

converge or to diverge an incident beam of light.

2. SI unit = m-1 or Diopter (D).

3. Power for a convex lens is positive. Power for a

concave lens is negative.

f in m f in cm

Find the power:

a) convex lens, f = 20 cm,

b) concave lens, f = -5 cm.

Example 1

Lens Formula

1

2

3

f = focal length

u = object distance

v = image distance

m = Linear magnification

hI = size of image

h0 = size of object

m < 1

m = 1

m > 1

36

1. An object is placed in front of a convex lens with focal length of 10 cm. Find the nature, position

and magnification of the image formed when the object distance is 15 cm.

37

Exercise 5.4

2. An object is placed 20 cm from a concave lens of focal length 15 cm. Calculate the image

distance. State the characteristics of the image formed.

38

3. A convex lens with focus length of 15 cm formed an image which is real, inverted and same

size with the object. What is the object distance from the lens?

39

4. When an object of height 3.0 cm is placed 20 cm from a concave lens of focal length 30cm,

what is the height of the image formed?

40

Applications of Lenses

Simple Microscopes

Application : to magnified the image

Lens : a convex lens

Object distance: less than the focal length

of the lens, u < f

Characteristics of image: virtual, upright,

magnified

The magnifying power increases if the focal

length of the lens is shorter.

41

OF F

object

eye

Application : view very distant objects like the planets and the stars.

Made up of two convex lenses :Objective lens and eyepiece lens

Focal length fo for objective lens is longer than the focal length for eyepiece lens, fe The objective lens converges the parallel rays from a distant object and forms a real,

inverted and diminished image at its focal point.

The eyepiece lens is used as a magnifying glass to form a virtual, upright and magnified

image.

At normal adjustment the final image is formed at infinity.

This is done by adjusting the position of the eyepiece lens so that the first real image

becomes the object at the focal point, Fe of the eyepiece lens.

Normal adjustment: The distance between the lenses is f0 + fe 42

Telescope

FoFo

Fe Fe

Light ray

from distant

object

Objective

lens Eyepiece

lensfefo

Final image

formed at infinity

u1 =

u2 = fe

Compound Microscope

43

Application: to view very small objects like microorganisms

Uses 2 powerful convex lenses (Objective lens, Eyepiece lens ) of

short focal lengths.

Focal length fo for objective lens is shorter than the focal length for

eyepiece lens, fe Object to observed must be placed between F0 and 2F0

Characteristics of 1st image: real, inverted, magnified.

object

Objective

lens

Eyepiece

lens

Final image

Fe

Fe

Fo

fo fe

Fo2Fo 1st image

fo < u1 < 2fo

u2 < fe

The eyepiece lens is used as a magnifying glass to magnify the first image formed by the

objective lens.

The eyepiece lens must be positioned so that the first image is between the lens and Fe, the

focal point of the eyepiece lens.

Characteristics of final image formed by the eyepiece lens: virtual, upright and magnified.

Normal Adjustment: The distance between the lenses is greater than the sum of their individual

focal length (fo + fe)

44