Jean Gorman, of Nayland College, asks :-

Why does light slow down in glass and what is the fundamental cause of the colour display in prisms and diamonds?

John Campbell, a solid-state physicist at the University of Canterbury, responded.

Light is an electromagnetic wave and its oscillating electric field therefore applies a force to any charged particle it encounters. All materials contain atoms and all atoms have electrically charged electrons orbiting the heavy nucleus.

When a beam of light enters a transparent solid, for example glass, the electric forces cause the electrons in orbit around the atoms to oscillate in step with the light wave. Now we have an accelerating charged particle so the atoms radiate, just like a radio-transmitter causes electrons to oscillate along a vertical wire which then radiates radio waves. The wave radiating from each atom has the over-all effect of slowing down the light wave travelling through the material.

As all materials have atoms, light travels slower in a material than it does in air or a vacuum. There is not much difference in the speed of visible light in all transparent solids and liquids. When compared to the speed at which light travels in the absence of atoms, they range from three-quarters speed (for water) to half speed (for diamond).

Because the molecules in a gas are about ten molecular diameters apart, light travelling in a gas is travelling mostly in empty space so has a speed which is not much different to that of light travelling in a vacuum.

The colour effect of a prism or rainbow (refraction) is due to another, but related, mechanism. We are quite used to the resonance of a struck bell, or stroked wine glass, or plucked guitar string. The electrons attached to atoms also have a natural resonance. This occurs at the frequency of ultra-violet (uv) light.

As we increase the frequency of the driving force towards the resonant frequency we reach a stage whereby the object being driven cannot keep in step with the driving force and it starts to lag behind. Two things happen. Energy is lost from the light beam (for example, the atmosphere protects us from high energy uv radiation) and the light beam slows. Hence blue light (higher frequency) travels slightly slower in transparent materials than does red light. Therefore a prism deviates (refracts) blue light more than it does red light.

Diamond is valuable not only because it is a rare material. It has the biggest difference in blue to red speed of all common transparent materials, so has the best colour display due to refraction.