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5.2UNDERSTANDING

REFRACTION OF

LIGHT

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Learning outcome

Explain refraction of light

Define refractive index as

determine the refraction index of a glassor perspex block

State the refractive index , n, as

describe phenomena due to refraction

Solve problems involving the refraction of

light

!

si

si

in

r

speed of light in vacuumspeed of light in a medium

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REFRACTIONOF LIGHT

Light can pass through transparent

materials.

If the two materials are of different

optical densities, light will change

direction at the boundary between

them.

This phenomenon is termed as the

refraction of light.

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IMPORTANTTERMINOLOGIES

i

r

NormalIncident ray

Ray in the

first

medium

Line

perpendicular tothe boundary

Angle of

incident

Angle between

incident ray and

normal

refracted rayRay in

the

second

medium

Angle of

refractionAngle betweenrefracted ray and

normal

Medium 1

Medium 2

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BENDINGOFTHE LIGHT

Normal

Air

Glass

Normal

Air

Glass

Light is not bent when it

travels along the normal

bent towards

the normal

bent away from

the normal

Enterdenser

medium

Enter less

densemedium

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REFRACTIVEINDEX

speed of light in vacuum

speed of light in a medium

Normal

Air

Glass

Light bends when travels

between two mediums with

different optical densities.

This is due to the change inthe speed of light.

Light travels faster in

optically less dense medium

and vice versa.

The refractive index , n, of a

medium is defined ass f li t i v c r ir

s f li t i t i

( )sin

sin ( )

v ci

r i

!

i

r

p

q

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THE LAWOFREFRACTION

Normal

Air

Glass

1.The incident ray, the

refracted ray and the

normal at the point of

incidence all lie in thesame plane.

2.The ratio of is a

constant, where i is theangle of incidence and r

is the angle of refraction.

( )s in

sin ( )

v a c u u mi

n r m e d iu m!

i

r

p

q

Snells Law

s i n

s i n

pn

q

!

sin

sin

i

r

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Worked Example pg 141

(a) The speed of light in glass is 2.0 x 108 m s-1 and the

speed of light in air is 3.0 x 108 m s-1. Calculate the

refractive index of the glass

(b) The refractive index of water is 1.3. What is the speed

of light in water if its speed in air is 3.0 x 108

m s-1

.

8

8

3.0 101.5

2.0 10glass

S peed of light in air xn

S peed of light in glass x! ! !

83.0 10

1.3

water

S peed of light in airn

S peed of light in water

x

v

!

!

therefore v = 2.3 x 108

m s-1

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Worked Example pg 142

(a)

Air

Clown

glass

Normal30o

r

Calculate the angle of

refraction in crown glass.

n=1.52

(b)

Air

water

Normal

60o

i

Calculate the angle of

incidence in water.

n=1.33

sinsin

glassinr

!

sin301.52

sin

o

r!

r = 19.2o

sin ( )sin

waterr airn

i!

sin601.33

sin

o

i!

i = 40.6o

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REAL DEPTHAND

APPARENTDEPTH

The legs appear to be shorter than

their actual length.

Why ?

Refraction

of light

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RAYDIAGRAM

Normal

1. Draw the

normal line

2. Mark the

position of

image

3. Two

diverging

lines from

image

4. Connect

ray line

from

surface of

water to

object

h

H

h = apparent depth , H = real depth

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RELATIONSHIPBETWEEN

REAL DEPTHAND

APPARENTDEPTH

Refractive index

Re ,

,

al depth Hn

Apparent depth h!

Apparent

depth, h /cm

Real depth, H /cm

O

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Worked Example pg 144

(a) Due to the refraction of light

(b)

h 10 cm

(c) Refractive index, n =H

h10

1.33h

!

h = 7.5 cm