Refraction

The phenomenon of refraction allows waves to be bent based on changes in the speed of the wave. The speed of sound waves shows us how sound waves are bent in air by changes in temperature.

Here we will take a look at refraction of light. The principal is exactly the same but the phenomenon is much easier to demonstrate with light.

Index of Refraction

The Index of Refraction is the ratio of the speed of light through that medium to the speed of light in a total vacuum. For example, Jean Bernard Foucault (1819 - 1868) measured the speed of light in water at about 2.26 x 108 m/s. This results in a ratio of about 1.33 when compared with the speed of light in a vacuum.

Light travels almost 2 1/2 times slower in diamonds than it does in a vacuum.

Here is a list of common mediums with their corresponding index of refraction.

Total Internal Reflection

When light travels from a medium with a high index of refraction to a lower index of refraction, it bend away from normal. With water, when the incident light strikes at an angle of 45.8 degrees, it is bent so far it actually travels along the surface of the water. Any light that strikes at a higher angle of incidence will not be able to penetrate the boundary and will be completely reflected. This phenomena is called Total Internal Reflection.

An interesting situation is created, therefore, when you look up from the bottom of a still pool. Because of the difference in the index of refraction of water and air, you are able to see in every direction, all within a field of view of only 90 degrees. You, however, will not be able to see anything but a reflection of the bottom of the pool outside of this field of view.

The field of view from inside a diamond is only 24.6 degrees. This explains why a diamond, when cut as a gemstone, glitters brilliantly when illuminated by a beam of light. The beam of light can be internally reflected numerous times without any loss of intensity before exiting from a facet.