Refraction of light

1 John says,

‘ When hunting a fish under water, you should aim your spear directly at the fish.’

Do you agree?

Yes, of course.

No, because the fish is actually located somewhere else.

No, because size of objects changes when they are put under water.

2 When sunlight falls on the water surface, which of the following occur(s)?

It is reflected back to the air.

It refracts into the water.

It is absorbed by water and turned into heat.

airglass

Refraction is the bending of light

when the light passes from one medium to another.

Introduction

airglass

Useful words to describe refraction of light

normal

incident ray

angle of refraction

angle of incidence

refracted ray

Introduction

airglass

• e.g. from air to glass

• Light is bent towards the normal.

incident raynormal

refracted ray

From a less dense to a denser medium

Introduction

airwater

From a denser to a less dense medium

• e.g. from water to air

• Light is bent away from the normal.

incident rayrefracted ray

normal

Introduction

sin i is directly proportional to sin r.

sin i

sin rO

Relation between angle of incidence and angle of refraction

i = angle of incidencer = angle of refraction

straight line passing through the origin

Laws of refraction

Video Simulation

airglass

incident ray

refracted ray

normal

airglass refracted ray

normal

• The incident ray,the refracted ray,and the normalall lie in the same plane.

Laws of refraction

i.e. sin isin r

= constant

• The ratio of the sin i to sin r is constant.

– This is called Snell’s law.

Laws of refraction

In general,

n1 sin 1 = n2 sin 2n1 sin 1 = n2 sin 2

sin isin r

• refractive index n =

sin asin g

ang=

a: air g: glass

• e.g. for glass,

where

Refractive index

Material Refractive index

Glass 1.5 – 1.7

Water 1.33

Perspex 1.5

Diamond 2.42

Refractive indices of some materialsRefractive index

Refractive index and speed of light distance that light travel in 1 s

distance that light travel in 1 svacuum (or air)

3 108 m

water (n = 1.33)2.25 108 m

glass (n = 1.5)2 108 m

diamond (n = 2.42)1.25 108 m

diamond (n = 2.42)1.25 108 m

3 Refractive index and speed of light distance that light travel in 1 s

distance that light travel in 1 svacuum (or air)

3 108 m

water (n = 1.33)2.25 108 m

glass (n = 1.5)2 108 m

diamond (n = 2.42)1.25 108 m

diamond (n = 2.42)1.25 108 m

Refractive index

Speed of light

fastestfastest

slowestslowest

4 Examples of refraction of light

a Bent chopstick

• The chopstick appears bent because of refraction

4 Examples of refraction of light

b Shallower in water• The depth that the object is

actually at is called the real depth.

O

Ireal dept

h

Simulation

4 Examples of refraction of light

c Flickering objects in hot air

• The object you see through the unstable hot air appears blurred and flickering.

airwater

Example 3A ray of light passes from air into water.Angle of incidence = 30

What is angle of refraction in water?nwater = 1.33

30

Example 3

Applying Snell’s law, 1.33 sin w = 1 sin 30 sin w = sin 30/1.33

w = 22.1

airwater

30

nw sin w= na sin a

= 0.376

22.1

Example 4A ray of laser hits a rectangular block.The table gives a set of results.

Notes: a = angles of incidence,g = angles of refraction.

(a) Is the refracted ray bent towards or away from the normal?

a

g 15 30 45 60 7511 18 28 34 40

TowardsTowards

Example 4(b) Find out how sin a and sin g are

related by plotting a graph of sin a against sin g.

a

g 15 30 45 60 7511 18 28 34 40

sina

sin g 0.259 0.500 0.707 0.866 0.9660.191 0.309 0.470 0.559 0.643

Example 4sin a

sin g 0.259 0.500 0.707 0.866 0.9660.191 0.309 0.470 0.559 0.643

0.2 0.4 0.6 0.8 sin g

sin a

0

0.20.40.60.81.0

0.62 0.20 = 0.42

0.94 0.30 = 0.64

slope = = 1.52 0.94 0.300.62 0.20

Example 4(c) Find the refractive index ng of the

glass block in used in the experiment.

Refractive index = = slope of graph sin a

sin g

= 1.52

The refractive index of the glass block used is 1.52.

Q1 A boy shines a torch under…A boy shines a torch under water as shown.

Which one shows the correct path of the light ray?

A Path X.B Path Y.

C Path Z.

D All of them.

Q2 A beam of light travels air to…A beam of light travels from air to water.

Which of the following is equal to the ratio sin r : sin i ?

A nwater : 1

B nair : 1

C 1 : nwater

D 1 : nair

waterair

normal

i

r

Q3 A light rays is incident on a…A light ray is incident on a glass prism.

Which equation can be used to find ? 30

20 40

D sin 40 = 1.5 sin

A =sin 30sin 20

sin sin 40

B =sin 30sin 20

sin 40sin

C = 1.5sin sin 40

Q4 True or false: When light is…

True or false: When light is incident on a surface along the normal, only refraction occurs; there is no reflection.

(T/F)

Q5 Statements:...

1st statement 2nd statementLight is bent towards the normal when it passes from air to glass.

Glass has a greater refractive index than air.

true? true? true? true?

YesYes YesYes

Q1 True or false: Light slows…

True or false: Light slows down when it enters a material from air.

(T/F)

Q2 True or false: The refractive…

True or false: The refractive index cannot be smaller than 1.

(T/F)

Q3 True or false: If light travel…

True or false: If light travel at the same speed in all materials, refraction would still occur when it passes from air to water.

(T/F)

Q4 True or false: If the speed of…

True or false: If the speed of light in raindrop is equal to that in air, there would be no rainbows. (T/F)

Q5 Sketch a ray diagram for the...

Sketch a ray diagram for the tip of the chopstick to show why the chopstick looks bent when dipped into water.