CHAPTER 9 REFRACTION OF LIGHT

REFRACTION OF LIGHT SELF LEARNING METHOD

Submitted by,

SUGANDHA SINGH

ROLL NO. 14162047

B.Ed 2014-2015

N A V R A C H A N A U N I V E R S I T Y

REFRACTION OF LIGHT

Topics to be covered:

a. Introduction

b. What makes the things visible

c. Changing the speed of light – refraction

d. Definition of refraction

e. Rules for refraction

f. Real life examples related to refraction

g. Experiment on rectangular glass slab

h. Laws of refraction

i. Snell’s law

j. Total internal refraction

k. Atmospheric refraction

l. Dispersion-

-In prism

The world is largely known through the senses. The

sense of sight is one of the most important senses. Through it we

see mountains, rivers, trees, plants, chairs, people and so many

other things around us. We also see clouds, rainbows and birds

flying in the sky. At night we see the moon and stars.

What makes the things visible...

Have you ever thought how se see various objects? You

may say that eyes see the objects? You may say that eyes see

objects. But, can you see an object in dark? It means that eyes

alone cannot see any objet. It is only when light from an object

enters our eyes that we see the objects. The light may have been

emitted by the object, or may have been reflected/ refracted by it.

Ever notice how your leg looks bent as you dangle them in the

water from the edges of a pool? Why do fish seem to radically

change position as we look at them from different viewpoints in an

aquarium? What makes diamonds sparkle so much?

These are all questions that can be addressed with the concept of

refraction. Refraction is the bending of light when it goes from one

transparent medium to another (e.g. air –to-glass or air-to-water).

This meeting place of two different media is called the interface

between the media. All Refraction of light occurs at an interface.

Changing the speed of light – refraction

Refractions occurs because the speed of light changes

(slow down) when the light enters a “denser” material.

In vacuum, light travels with a speed

c = 3.00 x 108 m/s

In a transparent medium, (ex. water or glass) the speed of light is

slower than its speed in vacuum.

WHY? Ans. interactions between photon and molecules of the

medium.

What does this have to do with refraction?

The difference in speed causes light rays to bend (refracted ) when

traveling across different transparent materials!!!

Refraction

• When light travels from one material to another it usually changes

direction

• The bending of light that occurs at the borderline of two materials is

called refraction

• the amount of bending depends on the optical properties of the two

materials --characterized by their index of refraction: n

• n is a number: n=1 for vacuum, n=1.33 for water, n=2.42 for

diamond, n=1.5-1.9 for different types of glass …….

• When the amount of bending is bigger, the difference in n is bigger

for the two materials .

• Geometrical concepts: incident ray, refracted ray, normal to the

point of incidence, incident angle, refracted angle.

Rules of refraction

Rule 1: When a light ray travels from a rarer medium to a denser

medium, the light ray bends towards normal.

Rule 2: When a light ray travels from a denser medium to a rarer

medium, the light ray bends away from normal.

Real life examples related to refraction

Activity 1:

Many times we see the thing which is differ from the original things.

Such as the star present in the sky is steady still they look like

twinkling. Moreover on read, when we look at farther distance it

feels like there is water on the road, but when we go near we see that

there is nothing expect the road.

You may have seen such things. Note them down.

Activity 1:

Observe these three images:

What is different in it…. Why does it happen?

Appearance of the coin inside the glass-when it is kept outside,

appearance of the coin slight up side then it is present in the glass,

appearance of pencil as if broken or bend in the glass filled with

water, for this various phenomenon, refraction is responsible.

Explanation of examples:-

1..

If you place a pencil in a glass of water, you will see that the pencil

appears to be bent. Now, you know for a fact that the pencil is not

bent at all, it just appears to be so. This is the effect if light. Light is

actually a form of energy called electromagnetic radiation. There is a

wide spectrum of radiation with X- rays, Ultraviolet Rays, infrared

rays etc. and most of them are present in the atmosphere with their

own definite wavelength and properties, but only wavelength that

we are able to see is that of visible light.

Since light is in the form of electromagnetic waves, it has many

characteristic properties like reflection, travels at different speeds in

different objects and most importantly, undergoes refraction.

2..

The term refraction is defined as the bending of light as it passes

from one type of materials into another. Because light travels at

different speeds in the two materials, it changes its speed at the

boundary of the two materials. If a beam of light hits this boundary

at an angle, then light hitting the side first will be forced to slow

down or speed up before light on the other side hits the new

materials.

This cause the beam to bend, or refract, at the boundary. Suppose we

were to place a coin in glass of water. The light bouncing off the

coin underwater, for instance, would have to first travel through the

water and the air to reach an observer’s eye. At the boundary, it gets

refracted and reaches the observer’s eye, thus appearing to be

slightly raised.

Angle of incidence- is the angle between a ray incident on a

surface and the line perpendicular to the surface at the point of

incidence, called the normal.

Angle of Refraction- The amount of bending that alight ray

experiences can be expressed in terms of the angle of refraction

(more accurately , by the difference between the angle of refraction

and the angle of incidence). A ray of light may approach the

boundary at an angle of incidence of 45°and bends towards normal.

Normal line- the lines bends away from normal line.

Experiment on Rectangular glass slab:--

Look at this rectangular slab, when the bent ray of light passes from

rarer medium like air to denser medium like glass meanwhile it

slightly bents at surface separating two mediums.

When light ray passes from rarer to denser medium it change it’s

direction at the surface separating mediums, this phenomenon is

called refraction.

Ray of refraction: The ray propagating through rectangular slab is

called ray of refraction.

Angle of refraction: the angle between the refracted ray and the

normal is called angle of refraction.

Incident ray- is a ray of light strikes a surface.

Reflected ray- A ray of light that is reflected from the surface of

glass slab.

Refraction :--

Higher speed (lower index of refraction)

Lower speed (higher index of refraction)

i

r

r

r

air

glass

Refracted ray

bends towards the

normal

Lower speed (higher index of refraction)

normal

Glass

Air

Refracted ray bends

away from the normal

normal

i

Laws of Refraction:

1. Incidence ray and refracted ray are on opposite sides of normal

and are in different mediums.

2. Whenever light ray enters from rarer to denser medium, it

slightly bents towards the normal and when it enters from denser

to rarer medium it bents away from the normal.

3. The ray incident normal to the plane is not refracted.

4. Incident ray, critical ray and normal to the plane are in same

plane.

Snell’s laws

• If light travels from material 1 with index of refraction n1 to material

2 with index of refraction n2 the following laws determine the

direction of the refracted ray:

The incident ray, the normal to the incidence point and the refracted

ray are all in one plane

1

2

1

2

Higher speed (lower index of refraction)

r

Total Internal Refraction

• At the border of two materials usually both reflection and refraction

appears

• In some peculiar situations however the refracted light is also

reflected! --> Reflection is total!

• This can happen when light travels from a medium with bigger

index of refraction to one with a smaller index of refraction, and the

incident angle is big enough

• for high enough a1= a0 --> a2=900

• if a1> a0 :no refracted ray --> total reflection

Total refraction in everyday life

• Atmospheric refraction

- The atmosphere made up of layer with different density and

temperature air

-These layers different index of refraction --> light refracted.

- distortion of the shape of Moon or Sun at horizon

- apparent position of stars different from actual one

1

2

1

2

- if light goes from layers with higher n to layers with lower -->

total refraction: -mirages, looming

• Light guides: optical fibers: used in communication, medicine,

science, decorative room lighting, photography etc…..

Dispersion

• Different color rays deflect in different manner during refraction:

violet light is deflected more than red…..

• By refraction we can decompose the white color in its constituents--

A prism separates white light into the colors of the rainbow: ROY G

BIV

• We can do the opposite effect too…..recombining the rainbow

colors in white light.

Atmospheric dispersion of light: rainbow (dispersion on tinny water

drops) or halos (dispersion on tiny ice crystals)

Refraction in Prism

Prism

Newton demonstrated in the 1600’s that white light passing though a prism

could be separated into its different colors. While at that time he believed in the

corpuscular theory of light, we know now that these individual colors represent

different wavelengths or frequencies. From our introduction to refraction it is to

be expected that light of different colors will bed through different angles. With

reference to Fig., white light travelling in medium n1 is incident at an angle i1

to the normal of one face of the prism having refractive index n2.

A prism can serve many purposes, including as a dispersive element, a beam

splitter and a polarizing device. Here we discuss the behaviour of a general

dispersive prism. as shown in Fig. The incident light is refracted at the first

interface and travels at angle t1 with respect to the normal. This light is incident

at the second face of the prism at an angle i2 and finally refracted again to exit

the prism at angle t2.

In prism light changes speed as it moves from one medium to another (for

example, from air into the glass of prism). This speed changes causes the light

to be refracted and enter the new medium at a different angle (Huygens

principle= is a method of analysis applied problems of wave propagation both in

the far- field limit and in near – field diffraction). The degree of bending of

light’s path depends on the angle that the incident beam of light makes the

surface, and on the ratio between the refractive indices of the two media (snell’s

law).

We know that in different medium light have different velocities. In some

mediums the velocity of light is as given in the below table:

No. Medium Velocity of light

1 Vaccum/ air 3,00,000 km/second

2 Glass (simple transparent

glass)

1,80,000 km/second

3 Water 2,25,000 km/second

In vacuum the velocity of light is Maximum . Whereas, in other

medium it is less. Thus, the velocity of light in different transparent

medium is different so when the light ray travels from one medium to

other medium its velocity changes. Therefore the refraction of light

occurs.

The refraction of light measured with the help of Refractive index of

light.

The ratio of velocity of light in vacuum to the velocity of light in medium

is called the absolute Refractive Index of that transparent medium.

It means that Absolute Refractive index of medium= velocity of light in

vacuum/velocity of light in medium.

Its symbol is myu (µ).

When the light ray enters from one medium to other medium instead of

absolute refractive index we consider its relative refractive index. For

example, in order to calculate the refractive index using formula for the

ray of light entering firm air medium to glass medium, then its relative

index can be obtained as follows:

Refractive index of glass with respect to water= velocity of light in air

(3,00,00 km/sec)/velocity of light in glass (1,80,000 km/sec)

Where, air (µ) glass= 3,00,000 km/sec

1,80,000 km/sec

So, air (µ) glass= 1.66

Where, air (µ) glass is called refractive index of glass with respect to

the air.

With the same method find the refractive index of water with respect to

air

Refractive index of water with respect to air=

When, air (µ) water=

So, air (µ) glass=

Exercise:

1. Define refraction.

2. Define refractive index.

3. What is unit of refractive index?

4. List out the factors on which the refractive index of a medium

depends.

5. Define angle of incidence.

6. State snell’s law.

7. State the condition at which Total internal reflection occurs.

8. What happens when a ray of light falls normally on the surface of a

mirror?

9. Does the light ray passes straight? Yes and No.

10. Through which medium the light ray passes in the glass slab

experiment?

11. What are the 4 Laws of Refraction.

12. Define angle if refraction.

13. Define angle of incidence.

14. Define the process of dispersion and how dispersion process your

daily life.

15. When does the relation of light takes place?

(A) When there is obstacle in the path of light.

(B) When the medium of path of light changes.

(C) When the light increases or decreases.