# Total Internal Reflection and Critical Angle

### Total Internal Reflection and the Critical Angle

1. In figure (a) above, the light ray is refracted away from the normal when moving from denser medium to less dense medium.
2. Figure (b) shows that, at a specific angle, the light ray is refracted 90o from the normal. It is refracted so much that it is only just able to leave the water. In such condition, the incident angle is called the critical angle.
3. The critical angle is the angle of incident in an optically denser medium for which the angle of refraction is 90°.
4. In figure (c), the light ray strikes the surface at an angle of incidence greater than c. There is no refracted ray; the surface of the water acts like a perfect mirror, and the ray is said to have been totally internally reflected.

#### The Equation Relates the Critical angle (c) with the Refractive Index

The critical angle can be calculated by using the following equation:

#### Requirements for Total Internal Reflection to occur.

1. The light ray must propagate from an optically denser medium to an optically less dense medium.
2. The angle of incident must exceed the critical angle.