Total Internal Reflection

Total internal reflection is the reflection of light incident at the boundary of a medium back into the medium whence it came. A varying proportion of the incident light may be totally internally reflected depending on the angle of incidence of the light and the critical angle of the medium.
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.

If the angle of incidence increases such that it is greater than c, all of the light will be totally internally reflected.

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

Reflection of Waves

Physics - Waves - Reflection
When a wave strikes a barrier, it is reflected. For simplicity, we will consider a wave striking a flat barrier. The wave will strike the barrier at a certain angle, and will be reflected at the same angle.

This article is part 1 of the series 'Reflection, Refraction and Diffraction'. Prior knowledge is not required. The link to part 2 of this series is at the foot of this article.

• Whenever we measure a wave being reflected off a barrier, we use an imaginary line Perpendicular (at 90 degrees) to the barrier to measure the angles from. This line is called the Normal. 
• The angle of incidence is the angle from the normal that the wave is incident at. It is abbreviated to i
• Using the rule 'the angle of incidence is always equal to the angle of reflection', we can see that the angle of reflection will be the same. The angle of reflection is abbreviated to r.

We measure from the normal rather than the surface to avoid complications. For example, imagine that a wave was being reflected at the very edge of a mirror - you would have no surface to measure from on one side! Also, as we will go on to explore now, the surface may be uneven.

The angle of reflection is still equal to the angle of incidence. Here is what is happening: each wave strikes a particular point on the surface which will have a particular gradient. We can find this gradient by drawing a tangent to the curve (the red line on the diagram above). This is the gradient at the one point on the surface that the wave hits. Now we can apply the same technique as above to find the angle of reflection. This is what is happening on any non-reflective surface, each wave is reflected off at a different angle to the ones nearby because the surface is so uneven - the waves dont just reflect off in random directions.

Next post in series: Refraction of Light
http://exploringgcses.blogspot.com/2012/01/refraction-of-light.html