What is total internal reflection and how does it relate to refraction?

Total Internal Reflection (TIR) is a phenomenon where light does not pass into another medium but instead gets reflected completely back into the original medium.

How it relates to refraction:

• Refraction happens when light passes from one medium to another (like glass to air) and bends due to a change in speed.
• But if light travels from a denser medium (like water or glass) to a less dense medium (like air), and the angle of incidence is large, something special can happen:
  1. At a certain angle (called the critical angle), the refracted ray travels along the boundary.
  2. If the angle is even greater than the critical angle, no refraction happens. Instead, the light reflects fully back into the medium — this is total internal reflection.

Conditions for TIR:

1. Light must travel from denser to rarer medium.
2. The angle of incidence must be greater than the critical angle.

Examples:

• Optical fibers use TIR to trap light inside.
• Mirages and sparkling diamonds are also results of TIR.

In short: Total Internal Reflection is a special case of refraction when all the light is reflected instead of refracted.