Physics - Waves - Reflection, Refraction and Diffraction
This article is part 3 of the series 'Reflection, Refraction and Diffraction'. Prior knowledge may be required. If you have difficulty with this article, read its precursor here: http://exploringgcses.blogspot.com/2012/01/refraction-of-light.html
Total internal reflection occurs alongside refraction, and only in the passage of light from a more optically dense material to a less optically dense material. When the light is incident at the boundary of the medium in this case, most of it is refracted, but a small proportion is reflected back into the medium.
The proportion of light reflected back remains small until the light is incident at an angle greater than the critical angle. Once this happens, all of the light is totally internally reflected. When a light ray incident at exactly the critical angle, the refracted ray runs perpendicular (at 90 degrees) to the normal.
The value of c depends not only on the optical density of the first medium, but also on the optical density of the medium that the light would be refracted to. In other words, it relies on the density of the materials either side of the boundary. This is important to remember.
The equation sin(c) = 1/n relates a medium's refractive index to its critical angle in air.
Total internal reflection has several uses. It can be used to carry optical signals along bundles of optical fibres because the light inside just reflects against the side continuously as it moves forwards. It is used in periscope prisms as well: because the critical angle for glass is 42 degrees, it will be totally internally reflected in a periscope arrangement because it will be incident to the boundary at 45 degrees, which is greater than c.
Read part 4 of this series, 'Diffraction', http://exploringgcses.blogspot.com/2012/01/diffraction.html
