What is total internal reflection?

Total internal reflection occurs when light is reflected back into a medium at an interface.

When light travels from a denser medium to a less dense medium, it bends away from the normal to the surface. This bending is known as refraction. The angle of refraction depends on the angle of incidence and the refractive indices of the two media. When the angle of incidence is increased, the angle of refraction also increases. At a certain angle of incidence, known as the critical angle, the angle of refraction becomes 90 degrees. At this angle, the refracted ray grazes along the surface of the less dense medium.

If the angle of incidence is increased beyond the critical angle, the refracted ray disappears and all the light is reflected back into the denser medium. This phenomenon is known as total internal reflection. The critical angle depends on the refractive indices of the two media and can be calculated using Snell's law.

Total internal reflection has many practical applications, such as in optical fibres, where light is transmitted through a core of high refractive index surrounded by a cladding of lower refractive index. The light is reflected back and forth within the core, allowing it to travel long distances without significant loss of energy. Total internal reflection is also used in binoculars and periscopes to reflect light and change the direction of the image.