What is meant by optical illusion? Give an example.

Hint: In order to solve this question we need to understand reflection and refraction. Reflection is defined as bouncing back of light from an interference in the same medium while refraction is defined as transmitting of light in another medium. Actually when a light is incident on interference then it undergoes two phenomena one is reflection and other is refraction. Also reflection are of two types one is regular reflection and other is diffused reflection.

Complete answer:
Optical illusion is defined as an illusion in perceiving any object due to bending of light.Look, we only can see and interpret only those things from which light is reflected or it must act as a source of light. So in refraction one light travels from one medium to another.Since the light speed changes in medium due to their refractive index, we sometimes see the apparent position object rather than the real position of it.

A common example is fish under water, if you view the fish from above the water then you would see an apparent position of fist at apparent depth which is less than real depth of water, this is an optical illusion. Optical illusion also occurs when the medium refractive index in front of an eye varies so light travels somewhere faster and somewhere slower so we see optical illusion.

Note: It should be remembered that if the light denser medium to rarer medium then it travels faster in rarer medium hence it bends away from normal so total internal reflection is defined as when the light from denser medium do not get transmitted in rarer medium rather it bounces back in same medium then in this case there would be complete darkness and we are not able to visualize objects.