When light is going from a less dense to a more dense medium, there is always some light refracted and some light reflected but when light is going from a more dense to a less dense medium, there is no refracted light if the angle of incidence is greater than a certain value.

This "special" angle of incidence is called the critical angle of incidence and its size depends on the refractive index of the two media.

The diagrams below illustrate this phenomenon.

The critical angle is the angle of incidence which produces an angle of refraction of 90°. (For glass in air the angle is about 42°.)

If we write "the refractive index of a medium = n", we mean the refractive index for light going from air (or a vacuum) into the medium. When considering internal reflection, we are thinking about light going the other way.

Snell’s law is often written as

where "i" means the angle between the light and the normal line in air and "r" means the angle between the light and the normal line in the medium. Hence, if we consider the situation where light meets the interface at the critical angle, c, we have

The critical angle and refractive index are different for different colours of light.

Practical uses of total internal reflection of light include for example in periscopes, binoculars, optical fibres (for telecommunication, click here for more detail) etc.

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