What is the critical angle in total internal reflection?

The critical angle is defined as the maximum angle of incidence at which light can strike the boundary between two mediums and still be completely reflected rather than refracted. When light attempts to pass from a denser medium (like glass or water) to a less dense medium (like air), there is a certain angle of incidence—above which all light reflects back into the denser medium, rather than bending away from it.

At angles less than the critical angle, some of the light will refract into the less dense medium, but once the angle exceeds the critical angle, total internal reflection occurs, meaning 100% of the light is reflected. This phenomenon is essential in various applications, such as optical fibers and prisms, where it's vital to retain light within a medium without it escaping. Understanding the critical angle is crucial to grasping how light behaves in different mediums and leveraging total internal reflection effectively.