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.

air glass

Refraction is the bending of light when the light passes from one medium to another.

Introduction

air glass

Useful words to describe refraction of light

normal

incident ray

angle of refraction

angle of incidence

Introduction

air glass

•  e.g. from air to glass

•  Light is bent towards the normal.

incident ray normal

From a less dense to a denser medium Introduction

air water

From a denser to a less dense medium

•  e.g. from water to air

•  Light is bent away from the normal.

refracted ray

normal

Introduction

⇒ sin i is directly proportional to sin r.

sin i

sin r O

Relation between angle of incidence and angle of refraction

i = angle of incidence r = angle of refraction

straight line passing through the origin

air glass

incident ray normal

air glass

normal

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

Laws of refraction

i.e. sin i sin 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 θ1 = n2 sin θ2

Material Refractive index

Glass 1.5 – 1.7

Water 1.33

Perspex 1.5

Diamond 2.42

Refractive index A measure of the speed of light in a particular substance.

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

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

Refractive index Speed of light

fastest

slowest

air water

Example A ray of light passes from air into water. Angle of incidence = 30° What is angle of refraction in water?

nwater = 1.33

30°

Example Applying Snell’s law, ⇒ 1.33 × sin θw = 1 × sin 30° ⇒ sin θw = sin 30°/1.33

∴ θw = 22.1°

air water

30°

nw sin θw= na sin θa

= 0.376

22.1°

Examples of refraction of light

a Bent chopstick

•  The chopstick appears bent because of refraction

Examples of refraction of light b Shallower in water •  The depth that the object is

actually at is called the real depth.

O

I real

depth

Examples of refraction of light c Flickering objects in hot air

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

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: If light travelled…

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

(T/F)

Q3 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)