Physics

# Critical Angle and Reversibility of Light

## Critical Angle:

When the ray of light moves from the dense medium to the rare medium, the value of the refraction angle ‘r’ is greater than the incidence angle i. When the value of the incidence angle is increased, a situation occurs when the value of the angle of refraction becomes 90°. The angle of incidence required for this value is called the Critical angle. This can be better understood from the following picture-

### Relationship between refractive index and critical angle:

That is, the refractive index of the rare medium relative to the condensed medium is equal to = 1 / critical angle.

The refractive index of a medium is lowest for red color and highest for the purple color, so the critical angle value is highest for red color and lowest for purple color.

Reversibility of light rays:

In the event of refraction, if a flat mirror is placed in front of the refracted ray, then this refracted ray will reflect and return to the exact path from which the incident occurred. This phenomenon is called the Reversibility of light.
It is clear from the picture that the ray of light C is refracting to the plane mirror A after refraction and again reflecting from the plane mirror and returning to the exact path it came from.
Again, Refractive index of glass relative to air-
On this basis, we can say that the refractive index of glass relative to air is the inverse of the refractive index of air relative to glass.

## Some practical examples of refraction of light:

### 1:Bent or broken look of a pencil placed in water on the surface:

A pencil placed in a glass of water appears bent at the point of contact of water due to refraction. In this situation, the rays of light coming from the end of the pencil immersed in water refract on the surface of the water, due to which the position of the last end of the pencil changes, due to this the pencil seems broken or bent.

### 2: Twinkling of stars at night in the sky:

Due to the change in temperature at night, the density of different layers of the atmosphere varies, due to which the refractive index of different layers of air also changes, due to which the rays of light coming from the stars stop intermittently. This is why the stars in the night sky seem to flicker because the light coming from them is constantly refracting, whereas the moon makes a bigger angle on the eye than the stars, so it does not seem to be twinkling.

### 3:Looking less than it’s the actual depth of the pond :

Due to the refraction of light on the surface of the water, seems the depth of the pond is less than its actual depth. When rays travel from denser medium to rare medium it bends away from the normal and the actual depth appears to be raised.

### 4:Higher appearance of the plane, the sun, stars, and the sky than actual:

Due to the heating of the Earth’s surface in summer, the refractive index of the air near it is high, and by the increasing height from Earth the refractive index decreases with decreasing temperature. Due to the refraction of light rays through the dense medium, the plane flying at high altitude, the sun, stars, and the sky appears higher than its actual position.

### 5:Visibility of the sun even below the horizon at sunrise and sunset:

Decreasing the heat when the surface goes up reduces the refractive index of the air, due to which the refraction of the light is repeated in the layers of the atmosphere, due to this, the sun is visible even below the horizon at sunrise and sunset.