Question

When light enters from denser medium to rarer medium at an angle greater than critical angle. It reflects back. This phenomenon is called:
A. Reflection at plane surface
B. Total internal reflection
C. Refraction at the plane surface
D. None of above

Answer 1

Hint: After seeing the above problem we remember one important Phenomenon. An optical phenomenon of the complete reflection of a ray of light within a medium such as water or glass from the surrounding surface back into the medium is called total internal reflection.

Complete answer: The condition of the total internal reflection is that the angle of incidence must be greater than a certain limiting angle, called the critical angle. This implies that the light must be travelling from the denser medium into the less dense medium for example glass to air.

here in the diagram the incidence angle must be greater than the critical angle.
Here in this question we see that light enters from denser medium to rarer medium at an angle greater than critical angle and then reflects back into the previous medium. So, this completes the condition for the total internal reflection.
Hence this phenomenon is known as the total internal reflection.
So, the correct answer is “Option B”.

Additional Information:
Importance of the total internal reflection: total internal reflection is important in fibre optics and applied in the polarizing prisms.
The word “total” in the “total internal reflection” is used because of the reason that all of the light that could possibly propagate away from this surface is reflected.

Note:
some important examples of the total internal reflection are:

A.)Sparkling of the diamond is due to the total internal reflection.
B.)Fibre optics cables transmit the information in the form of light which undergoes the total internal reflection during the propagation through the cable.
C.)Triangular prisms can be oriented in the position so that light hits at angle greater than the critical angle. That’s why here also total internal reflection occurs.
D.)Retro-reflectors are also the result of the application of the total internal reflection.