Define critical angle. Explain total internal reflection using a neat diagram.

The critical angle is defined as the angle of incidence in a denser medium at which the angle of refraction in the rarer medium (air) becomes 90°. When light travels from a denser medium to a rarer medium, the refracted ray moves closer to the surface. The specific angle of incidence at which the refracted ray just grazes the interface (i.e., makes an angle of 90° with the normal) is called the critical angle.

Relation Between Critical Angle and Refractive Index

The relationship between the critical angle (C) and the refractive index (μ) of a medium is given by:

Formula:
μ = ^{Sin(angle in air)} / _{Sin(angle in medium)}
At the critical angle, the refracted angle is 90°, so:
μ = Sin(90°) / Sin(C)
Since Sin(90°) = 1, the formula simplifies to:
μ = 1 / Sin(C)

Total Internal Reflection

Total internal reflection occurs when a ray of light traveling from a denser medium to a rarer medium meets the surface of separation at an angle greater than the critical angle. In this case, the light ray is reflected entirely back into the denser medium rather than refracted into the rarer medium.

Conditions for Total Internal Reflection

1. The light must travel from a denser medium to a rarer medium.
2. The angle of incidence in the denser medium must be greater than the critical angle of that medium.

Uses of Total Internal Reflection

• It allows light to propagate without energy loss, making it essential for optical communication systems.
• Optical fibers, which are used in telecommunications and medical imaging, operate based on this principle.