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Refraction of Light- Physics Guide for Class 8

Refraction of Light- Class 8 Science Guide

Information about Refraction of Light

Title

Refraction of Light

Class

Class 8

Subject

Class 8 Physics

Topics Covered

  • Refraction of Light 
  • Cause of Refraction of Light
  • Refractive Index
  • Rules of Refraction
  • Refraction of Light by a Glass Slab


Light normally keeps on propagating along a straight path in a given transparent medium. It, however, gets (significantly) reflected, when it falls on a shining or polished surface. The phenomenon of reflection of light is governed by the two laws of reflection.

It often happens that a light ray, propagating in one transparent medium, gets to enter, and then propagate, in another transparent medium.
For example, a light ray initially propagating in air, may fall on a glass block, or on the surface of water in a container.

We will now study, in some detail, the characteristics of the phenomenon of the passage of light from one medium to another. 

Refraction of Light 

When a ray of light, propagating (along a straight line) in one transparent medium, enters another transparent medium, it is observed to (usually) change its direction of propagation, or to 'bend its path. This observation is due to the phenomenon known as 'refraction of light'. Such observations are basically due to a change in the speed of light as it goes from one medium to another.

We usually regard Refraction as the phenomenon in which a ray of light, propagating obliquely in one (transparent) medium, changes its direction of propagation (or bends its path) when it goes into another (different transparent) medium. 

It is possible to observe refraction through a 'set-up' of the type described below. 

Activity 1
  • Take two coins.
  • Place one coin at the bottom of a beaker and the another alongside the beaker.
  • Now, half fill the beaker with water.
  • Next observe the two coins by looking vertically downwards from the beaker.
What do you observe about the position of the coin in the beaker compared with the coin kept alongside the beaker?
The coin in the beaker appears to be 'raised up' with respect to the coin alongside the beaker. 

Activity 2 
  • Dip a pencil in a glass of water at about 45° to the surface of the glass.
Examine the appearance of the pencil when viewed at an angle to the surface of the glass.
What do you observe about the appearance and shape of the pencil?
The pencil appears 'bent' near the surface of water.

  • All these activities (and many other similar phenomenon) can be easily understood in terms of refraction of light.
  • The phenomenon of bending of a light ray, when it goes (obliquely) from one transparent medium to another, is 'refraction of light'.
  • The phenomenon of refraction is related to many day to day effects in our life. 

Cause of 'Refraction' of Light

The speed of light is different in different media. In vacuum, light propagates at a speed of (nearly) 3×108 m/s (3 lakh kilometres per second). However, when its enters any other transparent medium, its speed decreases.
  • It is this change in speed of light that causes a light ray to bend when its goes from one medium to another.
  • Thus the bending of light rays is basically due to a change in the speed of light when it goes from one medium to another. 
  • The speed of light in a given medium is taken as an indicator of its optical density.
  • We say that the more is the speed of light in a given medium, the less is its optical density and vice-versa.
  • Vacuum (or free space or air) has the least optical density because the speed of light is maximum in vacuum
Of any two given media, the one having a higher value for the speed of light in it, is referred to as the optically rarer of the two media. The other, having a lower value for the speed of light in it, is referred to as the optically denser of the two media. Thus,
  • (higher optical density) optically denser medium ↔ lower speed of light
  • (lower optical density) optically rarer medium ↔ higher speed of light 

Representative Values of Speed of Light in Some Media 

Medium

Vacuum

Air

Water

(Ordinary) Glass

Diamond

(Approximate)

Speed of light (in lakhs of km/s)

3

≈3

≈2.25

≈2

≈1.25


Refractive Index

The ratio of the speeds of light, in a given pair of media, gives us a measure of the extent of refraction (or bending) of a ray of light as it goes from one of these media to the other. We use this ratio to define a term called the relative refractive index for a given pair of media.

Relative refractive index of medium 2 with respect to medium 1

The refractive index, or absolute refractive index of a medium (say, m), denoted by μm, is defined as follows:

Refractive Index and Optical Density

It is now easy to realise that the refractive index of a medium is an indicator, as well as a measure, of its optical density. The more the refractive index, the more is the optical density. 

Rules of Refraction

We can easily understand the observations, associated with the phenomenon of refraction of light, in terms of 'two' rules. We state them as follows:
  1. A ray of light bends towards the 'normal' when it goes (obliquely) from an optically rarer medium to an optically denser medium.
  2. A ray of light bends away from the 'normal' when it goes (obliquely) from an optically denser medium to an optically rarer medium.
We use these rules for understanding many phenomenon based on the refraction of light. Let us see how we can use these rules to understand the observations made in the Activities 1 and 2. 

Consider first the case of a pencil partially immersed in water.
The rays of light, starting from, say, the tip of the pencil, bend away from the normal as they move from water (optically denser) to air (optically rarer). To us, therefore, these rays appear to be coming from a point, that lies above the actual position of the tip of the pencil. The same is true for all other points of the pencil under water. The part of the pencil, under water, thus, appears bent with respect to its part above water.

Proceeding in exactly the same way, we can understand why:
  • a pond appears shallower (less deep) than it actually is.
  • a coin (put in an empty cup), not visible before, becomes visible after some water is poured into the cup. 

In the two ray diagrams, given below, we see that it is the bending of light rays (away from the normal) as they move from an optically denser medium (water) to an optically rarer medium (air), that is responsible for our observations.

Refraction of Light by a Glass Slab

A parallel faced glass slab can be easily used to study the basic features of the phenomenon of refraction. We can do some simple activities for this purpose. 

Activity 3
Go into a room which has thick curtains on its windows. Draw the window curtains and close the doors of the room. Let there be just one very narrow (pin-hole) opening through which a fine pencil of light enters the room. This pencil of light would be visible through the dust particles coming in its way. Now, take a thick rectangular slab of glass and introduce it in the path of this (visible) pencil of light.
We will observe that the path of pencil of light, coming out from the glass slab, gets displaced with respect to its original path.
This is because of bending of light as it goes from air to glass and then from glass to air.
(Note: The 'bending' may not be observed if the glass slab is not sufficiently thick.) 

We draw below the path of a ray of light as it moves through the two opposite parallel sides, AB and CD, of a rectangular glass slab.
We observe that, on refraction through the parallel faced glass slab ABCD,
  • the final refracted ray, RS is parallel to the incident ray, PQ. However, it is displaced, relative to the ray PQ, by an amount, d, say.
  • the incident ray, PQ bends its path and moves along the direction MN within the glass slab. 

A ray of light does not always bend its path when it goes from one medium to another. It keeps on moving along a straight path (i.e., goes undeviated) if it falls normally on the surface separating the two transparent media. There is still, however, a change in its speed.

Some Definitions

  • The initial ray PQ(M), falling on the glass slab, is known as the incident ray.
  • The path of the ray MN, within the glass slab, is known as the refracted ray.
  • The final ray (N)RS, coming out of the glass slab, is known as the emergent ray.
  • The angle, between the incident ray PQ(M) and the normal at the point of incidence (M), is known as the angle of incidence (∠i).
  • The angle, between the refracted ray MN and the normal at the point of incidence, is known as the angle of refraction (∠r).
  • The angles∠MNT' (=∠r') and ∠TNR (=∠e) are, similarly, the angles of incidence and refraction respectively, at the face CD of the glass slab. It turns out ∠e = ∠i. 
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