Wednesday, August 26, 2020

Total Internal Reflection

 When light passes from denser medium to rare medium it bends away from normal after refraction. At certain value of angle of incidence angle of refraction becomes 90o or right angle. The angle of incidence in denser medium when angle refraction becomes 90o is called critical angle. If angle of incidence is greater than critical angle the light reflects back in denser medium. This phenomenon of reflecting light in same medium instead of passing to another medium is called total internal reflection.

Examples of Total internal reflections are

1. Light Pipe

2. mirage

Q. how light passes in curved path inside light pipe?

Light pipe is made of high refractive index glass compared to air. If incident light is passed with an angle greater than its critical angle light ray undergo multiple total internal reflection as shown in fig below.

Q. What is mirage? How is it formed?

The mirage is caused by the total internal reflection of light at layers of air of different densities.In a desert, the sand is very hot during day time and a result the layer of air in contact with it gets heated up and becomes lighter. The lighter air rises up and the denser air from above comes down.

As a result, the successive upper layers are denser than those below them. A ray of light coming from a distant object, like the top a tree, gets refracted from a denser to a rarer medium.

Consequently the refracted ray bends away from the normal until at a particular layer, the light is incident at an angle greater than the critical angle. At this stage the incident ray suffers total internal reflection and is reflected upwards.

When this reflected beam of light enters the eyes of the observer, it appears as if an inverted image of the tree is seen and the sand looks like a pool of water.




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