If in the process of refraction angle of incidence is increased to more than the critical angle when the ray of light passes through the dense medium, then the ray of light is fully reflected from the plane separating the two mediums and returns back to the same medium. Is called the Complete or Total Internal Reflection of light. This can be understood in a better way than the following picture-
Conditions for Total Internal Reflection:
For total internal reflection, it is necessary that the light must pass through the dense medium to the rare medium and the angle of incidence should be greater than the value of the critical angle.
Examples of Total Internal Reflection in Daily Life:
1:Shiny Diamond Appearance:
The refractive index of diamond is 2.42 with respect to air and the critical angle for diamond-air is 24 degrees. Which is very less, so when a ray of light enters the diamond, it is not easily able to come out of its inner plane due to repeated internal reflection. Looks very bright.
2: Shining of Cracks Lying in a Glass:
A thin layer of air is formed in the cracks of the broken glass, due to which the cracks begin to appear bright, the reason is that the sinuous light on these cracks does not go into the layer of air and is completely changed from the separated surface of glass and air Goes brightly when seen.
3: Air Bubble in the Paperweight:
The light bubbles in the air bubble inside the paperweight and on the separate surface of the glass will face at an angle greater than the critical angle, causing complete internal reflection of the light, which makes the bubble appear bright.
4: Mirage in the Desert:
In the desert, in summer days, sometimes waving at some distance, reflecting reflections of the reservoir or trees located far away, this visual illusion is called Marichika. The reason for this phenomenon is the complete internal reflection in the layers of air. Because the sand in the desert is very hot on hot days, the air near it also acts as a rare medium than above, so the light coming from above through some object enters the rare medium. If the angle of incidence is greater than the critical angle, there is a complete internal reflection of the light beam. The reflected beam tilts towards the normal due to the dense medium above, and for the distant observer, the light appears to be coming from the bottom of the ground surface causing the illusion of water at the site.
5: Brightness of test tube in water:
When the empty test tube lying in a beaker filled with water is kept slowly curved, the incidence angle of light incident on it increases. When this angle exceeds the critical angle (41 °) of the glass-air, the light is not able to enter the air inside the test tube, but instead reaches the eye by reflecting the entire interior of the test tube wall, causing the surface of the test tube to look bright as silver starts giving but when water is filled in the test tube, the critical angle for the glass-water becomes much larger than the critical angle of the glass-air, in which case the complete internal reflection of light is not possible and the test tube does not appear bright.
6: Mirage in cold countries:
The occurrence of Marichika in cold countries is unlike that of Marichika in the desert because the air near the ground in colder countries acts as a dense medium than above. Due to which the rays of light from the ship floating on the seawater are refracted by the layers of air going upward and away from the normal and full internal reflection takes place at a particular layer. After reflection, these rays are refracted by different layers of air and tilt towards the normal. When it reaches the observer’s eye, it appears to be a ship floating in the sea, hanging in the sky. It is called the Marichika of cold countries.