You might have seen a straight rod kept inclined partially in water, appears to be bent, at the surface of separation. It is due to refraction oflight.

When light rays travels from one medium to another at an angle, it changes the direction of path at the interface of the two media. This phenomenon of light is known as refraction.

Consider the refraction through a parallel sided glass slab. Here i is the angle of incidence and it is the angle between incident ray and normal. Angle of refraction r is the angle between refracted ray and normal. Trace the path of incident ray and refracted ray for different angles of incidence by using pin and pencil. Tabulate the readings in the table below.

On analysing the results what do you observe?

(i) The incident ray, the refracted ray and the normal to the interface at the point of incidence, all lie in the same plane.

(2) The ratio of sine of the angle of incidence to the angle of refraction is constant. This law is known as the law of refraction or Snell's law.

ie.., sinisinr= constantc

Refractive index is the relative property of two media. If the medium carrying incident ray in vacuum, then the ratio sinisinris called the absolute refractive index of the second mediwn. The relative refractive index of any two media is equal to the ratio of their absolute refractive indices. Therefore if the absolute refractive indices of media 1 and 2 be n1 and n2 respectively, then the refractive index of medium 2 with respect to medium 1.

sinisinr=1n2=n2n1

The refractive index of medium 1 with respect to mediwn 2

2n1=11n2

If lhe light travel through three media of refractive indices n1, n2and n3 then,

2n3 = 1n3 x 2n1

lf 1n2 > 1,r < i i.e., refracted ray bends towards the normal. The medium 2 is aid to be optically denser than medium 1. On the  other hand 1n2 < 1, r > i, the refracted ray bend away from normal.

During refraction, the speed, wavelength and the intensity of light changes, while frequency remains the same.

 Refractive index

### Why does Refraction Take Place?

Refraction of light occurs because the speed of light is different in different media and light has a tendency to cover distances in least time. By bending at the interface light ray taks a shorter path. If v1 and v2 are the speed of the light in first media and the second media respectively.

sinisinr=V1V2

The refraction is responsible for many interesting optical phenomea. An object placed in a denser medium, when seen from rarer medium, appears to be raised.

It can be shown that the apparent depth is real depth divided by the refractive index.  The sun is visible before the actual sunrise and a little after sunset.

By the actual sunrise, we mean the actual crossing of the horizon by the sun. Fig shows the actual and apparent positions of the sun with respect to horizon. Refractive index of air with respect to free space is 1. 00029. Due to this, apparent shift in the direction of the sun is about half a degree ( l/2 °) and the  corresponding time difference between actual sunset and apparent sunset is about 2 min.

The apparent flattening of  the sun at sunset and sunrise is also due to the same phenomenon.