Give a demonstration for total internal reflection. What are the technological applications of total internal reflection in nature? Briefly explain it.

Hint: Total internal reflection is an example of light passing from one medium to another. If the incident angle is greater than the critical angle of incidence for the combination of mediums then instead of passing through the other medium, the ray of light reflects back to the same medium. There are artificial applications in fibre communications.

Complete answer:

In the above figure we are considering two mediums ${M_1}$ and ${M_2}$ with a critical angle of reflection,${I_C}$ (shown in thin dashed line). Now generally a ray of light passes from one medium to another medium, it bends away or towards the normal (shown by the thick dashed line above) based on the refractive indices of the mediums. Consider an incident ray moving from ${M_1}$ to ${M_2}$, then if the second medium is rarer, then the refracted ray will bend away from the normal (shown by blue rays). Now for certain cases if the ray is incident at an angle more than critical angle then the ray will not pass the second medium and instead reflects back to the same medium (shown in red rays).
Technological applications - This concept of total internal reflection is widely used in fibre optics communications. There may be much more detailed and complex working of the optic cables. But we will discuss a rough way of working using the concept of internal reflection. Generally, the total internal reflection works when a ray of light passes from a denser medium to a rarer medium. In an optical fibre, the core is made up of a particular glass material and the outer surface is coated by another glass material based on the critical angle of the combination. When a ray of light passes through the glass, it reaches the interface on the way and since the angle is set more than the critical angle, it reflects back to core glass material and moves ahead in the fibre. This happens all the way long through the cable. In the figure shown below, a cartoon of the rays (in red) reflecting internally is shown to move from left to right.

Note: In the first diagram, the colours of the rays are simply colours to represent two cases of incident rays and does not mean different wavelengths. Although different wavelengths have different refractive indices. Other applications include the use of fibres in the field of medical surgeries and as ornamental decorations.