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Colors » Science Behind Colors » Physics » A closer look at reflection and refraction: Total internal reflection

A closer look at reflection and refraction: Total internal reflection

It has been established previously that when light enters a transparent medium, it is refracted, and not reflected. But on closer inspection, this is not quite true. When a light ray emanating from an underwater source exits the water, it is refracted in accordance to the laws of refraction. But at the same time, a weak reflected ray can be detected within the water, and this ray follows the law of reflection.

One of the most interesting questions that comes to our minds when we digest the refraction theory is, what happens if the refracted ray bends so much, it cannot exit the medium? In fact, this question is a valid one—and the answer has been given the name total internal reflection.

When a ray of light exiting a glass is refracted, it will emerge at the angle of refraction decided by the refractive index ‘n.’ But at a certain angle of incidence, the refracted ray may coincide with the edge of the glass block, that is, the angle of refraction is 90o precisely:

The value or magnitude of that certain angle of incidence is known as the critical angle ‘c.’ Of course, the laws of refraction will all still be obeyed, but the ray will move along the edge of the glass only.
If, however, the angle of incidence exceeds the critical angle, it is logical to expect that the refracted ray, will, in fact, be retained within the medium. This means that the ray entering the glass block at an angle greater than c will not be seen to emerge. Now remember that there is also a reflected ray being produced within the glass block. However, as this ray remains in the block as well, all the light is internally reflected, and total internal reflection occurs.

This phenomenon has been put to a great deal of use in modern technology, with one of the most prominent uses being the basis of fiber optics. ‘Fiber optics,’ (a term used to describe the combination of several thousand very thin glass fibers along which light may be piped), is used to make the very latest kind of telephone cables, in which the conventional cable is replaced by very pure or optical glass strands or fibers which carry signals along them in the form of laser light pulses. The curved fibers retain the laser light within them using the same principle of total internal reflection.

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