The light is bent when it passes through the

glass block

A pencil immersed partially in the water

appears bent.

These phenomena are caused by refraction of lightHow ?

A glass block

A ray box A light ray

REFRACTION OF LIGHT

Definition

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Natural phenomenon

Law of refraction

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Solving problems

Formulae

REFRACTIONRefraction of light is the bending ( change direction) of a light

rays at the boundary as the ray travel from one medium to another

>

>>

air

glass

Incident ray

Refracted ray

normali

r

i = angle of incidencer = angle of refraction

Emergent ray

( due to the change of velocity )

>

>

>

air

glass

Incident ray

Refracted ray

normali

r

The light ray is seen refracted towards the normal ( the ray travel from less dense medium → denser medium

eg: air → glass

The light ray seen refracted away from the normal ( the ray travel from more dense medium to less dense medium

eg: glass → air

Original path

Laws of refractions

rin

sinsin

Known as Snell’s law

Where n = refractive index of the medium

n represent the denser of a medium

The bigger the n, the bigger the effect of the bending of

light

Sin r

Sin i

Gradient = nA

Medium A

Medium B

density of medium A > medium B

Gradient = nB

>>

<

_

>

<

<

The pencil appears bent because of

the refraction of light

Apparent depth, h

Real depth , H

normal

depthapparentdepthrealn

Examples:Calculate the refractive index of the following;

• n = sin θ (vacuum@air) sin θ (medium)

= sin 48/sin 15 = 2.87

• n = sin θ (vacuum@air) sin θ (medium)

= sin 60/sin 40 = 1.35

42o

15o

30o

50o

Perspex

AirAir

Glass

Refractive Index, n and Speed of Light

• The speed of light in vacuum, c = 3.0 x 10 8 ms-1 > (constant) • Light travels much slower in transparent materials.• The refraction of light is due to this change of speed.

Refractive index, n = Speed of light in vacuum @ air = c Speed of light in medium v

Velocity of light in air is higher

Velocity of light in water is lower

Formulae to determine the value of n are:

rin

sinsin

depthapparentdepthrealn

mediuminlightofspeedairinlightofspeedn

Watch this movies!

The source of the light

The source of the light

The ray is reflected through the first medium

Total internal reflection is occur..

1st medium

2nd medium

Total internal reflection

is a phenomenon occurs when :

a) the light ray travel from an optically denser medium to a less dense medium .

b) The angle of incidence must be greater than the critical

angle , that is, i > cnormal

i

900

water

air

c

i > c

Total internal reflection

water

air

cn

sin1

cc 1sin 1

Normal

Ray box

Semi-circular block glassi

r

i = 42o

r = 90o

i = Critical angle, c = 42o

Refractive index, n = sin ØV sin Øm

n = sin 90 = 1 sin c sin c

n = 1/(sin 42o) = ……………

Tangent

when r = 90o

What happen if i is bigger than c?

The light is reflected internal at the surface in a medium (glass).

Its known as Total Internal Reflection

Normal

Ray boxTangent

State two conditions for total internal reflection could occur.

ci

Glass: high density

Air: low density

i) Light ray passes from a denser medium to a less dense mediumii) The angle of incidence (in the denser medium) is greater than the

critical angle.

More examples of natural phenomena involving total internal reflection:

mirage

Sparkle of diamond

Rainbow formation

Sky

Cool air

Hot air

Image of the sky

Total internal reflection

Road surface

1. In hot days, a person travelling in a car will see an imaginary poolof water appearing on the surface of the road.

2. The layers of air near the ground are hotter and less dense. The layers higher up are cooler and denser.

3. Light ray from the sky bends away from the normal.

4. The angle of incidence increases until it reaches an angle greater than the critical angle.

5. Total internal reflection occurs and the light is reflected upward.

6. 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.

1. The critical angle of diamond is about 240 which is much smaller than the critical angle of glass which is about 420.

2. When light rays enter the diamond from the top facets, most of the light rays are totally reflected at the bottom facets.

3. The diamond should be carefully cut so that the angles between the cut facets are very precise to prevent light from escaping from the sides or bottom of the diamond.

4. The sparkle of the diamonds is mainly due to the dispersion of white light into different colours.

Total internal reflection

dispersionrefractionRed

violet

1. When sunlight shines on raindrops, refraction and total internal reflection of light occur in the raindrops.

2. When an observer receives the refracted light from the raindrops at a specific angle, a vision of rainbow is formed.

sunlight