5.2 REFRACTION OF LIGHT

PREPAIRED BY:PHYSICS DEPARTMENT

SBPI TEMERLOH

Situation related to refraction of light

• A straw appears bent or broken when it is partially immersed in water as shown

• This situation show the effect produced when light travels from one medium to another

• The effect are due to change of direction of light when light travel from one medium to another

Refraction of light• Refraction of light is phenomenon

where the direction of light is change when it crosses the boundary between two materials of different optical density

• Light rays bend towards the normal when the rays travel from a less dense medium to a denser medium (from air to glass)

• This means, the angle of refraction, r is smaller than the angle of incidence, i which r < i

• Light rays bend away from the normal as it travels from a denser medium to a less dense medium

• Light rays travel much slower in a denser medium. When light ray travels from one medium to another, its speed changes. The change in speed of the light ray causes the change of it direction.

Laws of refraction• When a ray of light travels from one medium into

another,

(i) the incident ray, the refracted ray and the normal all lie in the same plane(ii) the ratio of the sine of the angle of incidence (sin i) to the sine of the angle of refraction (sin r) is constant;

This is also known as Snell’s law

constantrsin isin

Refractive index• When a ray of light travels from the air to a transparent

material, the constant is known as the refractive index, n of the medium

• The refractive index is unitless• The refractive index for a few media is shown below

rsin isin

rsin isin nindex, Refractive

Material Refractive index

VacuumAir

GlassIce

Water

1.001.0003

1.501.311.33

1. Calculate the refractive index of the glass.

2. Calculate the refractive index of the glass.

5.128sin45sinrsin i sin

0

0

n

n

n

347.140sin60sin

0

0

n

n

Refractive index and the speed of light

• Light travel at a very high speed of 3 x 108 m s-1 in a vacuum

• When a ray of light passes from air or vacuum into a medium which is optically denser, its speed is reduce

• The bending effect of light when it travels from air into a medium depends on:(i) the type of the medium(ii) the angle of incidence

• Refractive index is given by:

• The speed of light in different materials varies and thus the refractive indices of material are different

vmedium, ain light of speedcin vacuum,light of speed n

• The value of n>1 for all materials because the speed of light in a vacuum is larger than the speed of light in the materials

• Any material with high refractive index is said to be optically denser then material with a lower refractive index

Example 1• Calculate the index of refraction for light traveling from

air to a medium with the speed of 1.8 x 108 m s-1. [ speed of light in air = 3.0 x 108 m s-1]

Solution:

Example 2Calculate the angle of refraction, given that nglass =1.5

Solution;

0

0

0

00

36.16r 1.5

25sin sin

25

6590 incidence, of angle

r

i

Real depth and apparent depth• Refraction of light in water enables us to look round a

corner as shown below

• A coin is put in beaker without water at first, we cannot see the coin because light rays from the coin cannot reach our eyes

• When the beaker is filled with water, we can see the coin because light rays from the coin are refracted towards our eyes

(a) Light rays from the coin cannot reach the eye

(b) Light rays from the

coin reach the eye

• Similarly, any object below a glass or at the bottom of a pond appears to be raised from its real (actual) depth. This apparent depth is caused by the refraction of light

• (a) light rays from the object are refracted away from the normal as they leave the glass or water surface(b) when produced backwards, these rays meet at I where the object can be seen as though it is there(c) The apparent depth,d is the distance of the virtual image,I from the glass or water surface(d) The real depth,D is the distance of the object,O from the glass or water surface

• In optical astronomy, the ‘twinkling’ of stars is due to the refraction of light

(a) the earth’s atmosphere is made up of layers of gases with varying optical density and

refractive indices(b) as light from a star passes through those layers of gases, it experiences repeated refraction(c) this causes scintillation (twinkling) which smears the apparent point-like image of a star into a disc.

More to know• Figure shows that when fish sees a

butterfly, the butterfly seems to be farther away when viewed by the fish.

• The actual position of the butterfly is at O but the position of the butterfly is at I

• This is due to the refraction of light at the surface of water

Relationship of refractive index to real and apparent depth

• The diagram show light ray from object, O is refracted at point P on the surface of water. The image is observed to be situated at I.

Solution :

ddepth,apparentDdepth,realnhence,

INON

IPOP

OPPNIPPN

rsin isin nindex,Reractive

Example 1Some coins are found lying at the bottom of a pond of water 1.4 m deep. What is the depth of the coins when viewed from above the water surface?[refractive index of water = 4/3]

Solution:

1.05m34

1.4n Ddcoin, theofdepth apparent thus,

ddepth,apparent Ddepth, realn water,ofindex refractive

Example 2The real depth of a swimming pool is 4.20m. If the refractive index of water is 1.33, what is the apparent depth of the swimming pool?

Solution:

m 3.16 depth Apparent

depthapparent 4.201.33

depthapparent depth real waterofindex Refractive

The End