Refraction of light

What is refraction of light?

Light is a form of electromagnetic wave. It travels in a straight line until encounters an obstacle or there is a change in medium. When a ray of light enter from one medium to another medium, it deviates from it's path. The change in path of light due to change in medium is called as refraction.

The image below shows the refraction of light. When a ray of light traveling in medium 'A', here the medium is air, enter in medium 'B' , second medium is glass, it deviates from its path. The change in path is due to the change in speed of light.

Why is the refraction of light happens?

When the ray of light enters into a denser medium it's a speed decreases, light takes more time to travel same distance compare to the first medium. To minimize the distance traveled in denser medium light bends towards the normal. If the ray of light enters into a rarer medium compared to the first medium, it's speed increases, light can cover more distance easily in rarer medium so it bends away from the normal.

We can summarize this in two lines.

When a Ray of light enters in a denser medium it bends towards the normal.
When a Ray of light enters in a rarer medium it bends away from the normal.

What is angle of incidence and angle of refraction?

The angle between incident ray and normal is called angle of incidence and the angle between refracted ray and normal is called angle of refraction. The image angle of incidence is in air and angle of refraction is in glass.

If second medium is denser, angle of refraction will be smaller than the angle of incidence and vice-versa.

The image above shows small angle of refraction as glass is optically denser than air.

Laws of refraction

There are two laws of refraction. First law of refraction is similar to second law of reflection and second law of refraction is known as Snell's law.

i) According to first law of refraction, " incident ray, refracted ray and normal lie in same plane".

ii) According to second law of refraction, " the ratio of sine of angle of incidence ( simply Sin i) to the sine of angle of refraction (simply Sin r) is equal to refractive index of medium" in which light enters.

As the refractive index of one medium remains same so second law of refraction can also be states as, "the ratio of sine of angle of incidence ( simply Sin i) to the sine of angle of refraction (simply Sin r) is constant."

sin i/ sin r = n

Where n is refractive index of medium

What is refractive index

Refractive index is the measure of optical density of one medium with respect to other medium. The change in speed of light depends on the optical density of medium in which its entering. If the density of medium is high, speed decreases and angle of refraction becomes smaller. It means higher density ratio between second and first medium leads to greater change in speed of light and smaller the angle of refraction. Similarly, If the density of medium, in which light is entering, is low, speed increase and angle of refraction becomes larger. It means low density ratio between second and first medium leads to increase in speed of light and greater angle of refraction.

Refractive index formula

Refractive index is ratio of speed of light in first medium to speed of light in second medium. Let speed of light in medium one is v1 and in medium 2 is v2, and refractive index is n

Refractive index n= v1/ v2

Here medium 1 can be anything, like water, air, glass etc.