What causes total internal reflection?

Total internal reflection occurs when light travelling in a denser medium hits a less dense medium at an angle greater than the critical angle.

Total internal reflection is a phenomenon that occurs when a propagating wave strikes a medium boundary at an angle larger than a particular critical angle with respect to the normal to the surface. If the refractive index is lower on the other side of the boundary and the incident angle is greater than the critical angle, the wave cannot pass through and is entirely reflected.

The critical angle is the angle of incidence above which the total internal reflection occurs. This is the angle at which light is refracted to an angle of 90 degrees with respect to the normal. If the angle of incidence is greater than this angle, the light cannot escape the medium, and instead, it is reflected back into the medium. The critical angle can be calculated using Snell's law by setting the refraction angle equal to 90 degrees.

The phenomenon of total internal reflection is not only observed with light but also with other waves such as sound and radio waves. It is the principle behind devices like optical fibres and prisms. In optical fibres, light signals are transmitted with very little loss because of total internal reflection. Similarly, in prisms, total internal reflection can be used to reflect light, or to split light into its constituent spectral colours.

In summary, total internal reflection is caused by the combination of the angle of incidence, the refractive indices of the two media involved, and the critical angle. When light travels from a denser to a less dense medium at an angle greater than the critical angle, it is completely reflected back into the denser medium. This phenomenon is fundamental to many optical devices and technologies.