10th Class Science Light - Reflection and Refraction Refraction of Light

        Refraction of Light

We know that light always travels in a straight line and never changes its direction. But this fact is true as long as it travels in one medium. As soon as it changes its medium i.e. as it goes from one medium to another, it changes its direction at the border of the two medium. This change in the direction of light as it goes from one medium to another is called refraction of light.

For example, when light travels from air to glass, glass to water, water to air, etc it undergoes refraction. Let us consider a rectangular glass slab ABCD as shown in the figure given below. A ray of light PQ is incidented on the glass slab from air at the point Q. The incident light passes through the glass slab. Since the glass slab is optically denser than air, so the ray of light changes its direction as it goes from air into the glass along the line QR, inside the glass slab.

This ray of light when emerges from the glass slab along RS, again it changes its direction, because of the difference in the density of both the medium. Here PQ is incident ray, QR is refracted ray and RS is emergent ray. The incident ray and the refracted ray are not along the same direction. \[{{N}_{1}}Q{{N}_{2}}\]and\[{{N}_{3}}R{{N}_{4}}\]are the normal at the point Q and R to the surface of the glass slab. The angle, between the incident ray and the normal is called angle of incidence and the angle, between the refracted ray and the normal at that point is called angle of refraction. The angle of incidence and angle of refraction are not usually same.

The real cause for the refraction of light is the difference in the density of the two medium. Due to difference in the density of the two medium, the speed of light changes as it goes from one medium to the another, and hence refraction takes places. The medium, in which the speed of light is more of the two medium, is called the rarer medium and the medium, in which the speed of light is less is called the denser medium. When the ray of light goes from rarer medium to the denser medium it bends towards the normal; and when it goes from denser medium to the rarer medium, it bends away from the normal. The distance between the emergent ray and the original direction of the ray of the light is called the lateral displacement.

              Various Effect of Refraction of Light

             A Pencil Appears Bent when Partially Immersed in a Glass of Water

When a pencil is partly immersed in water and held obliquely to the surface of water, it appears bent at the point where it enters into the water. This apparent bending of the pencil is due to the refraction of light as it passes from air into the water is due to the refraction of light.

              The Swimming Pool Appears to be Less Deep than Actually it is

The apparent depth of the water in the swimming pool appears less than actually it is, because of refraction of light when it passes from air into the water. When we look into the pool from above, the ray of light converges above the base of the pool; and hence it appears to be less deep, than actually it is.

              A Coin Placed under Water appears to be raised

A coin appears to be raised from the base, when placed in a glass of water. It is due to the refraction of light that takes place when it goes from water into air. When the coin is place under water, due to refraction of light a virtual image is formed near the surface of the water. Since the virtual image of coins which we see is near the surface of water, the coin appears to be raised.

             Laws of Refraction

When the light travels from one medium to another, it undergoes refraction according to the following law:

• The incident ray, the refracted ray and the normal, all lies in the same plane.
• The ratio of sine of angle of incidence to the sine of angle of refraction, is always a constant for the two given medium, in which refraction takes places.

\[\frac{\text{Sin}\,\text{i}}{\text{Sin}\,\text{r}}\text{=}\,\text{constant}\,\text{=}\,\text{ }\!\!\mu\!\!\text{ }\]

This constant is called the refractive index of the medium. It is also known as the Snell’s law. The refractive index of a medium gives an indication of the light bending ability of that medium. For example, the refractive index of glass is more than that of water, hence light rays bend more on passing from water into glass. When light goes from one medium to another, the value of refractive index is called the relative refractive index. If the light is going from vacuum to another medium, the value of the refractive index is called the absolute refractive index. An object, having higher refractive index is optically denser than another object, having lower refractive index.