Malcolm Bignell, of Queen Elizabeth College, asks :-

Why does sound travel faster in solids than in air, whereas light travels slower?

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

Two quite different mechanisms are involved. Sound travels by atoms and molecules bashing against their neighbours. In a solid or liquid the atoms and molecules are touching their neighbours which is shown by the fact that we cannot easily compress solids or liquids. Thus it is easy for them to pass on the extra vibrations due to sound. If we turn a volume of solid or liquid into a gas the molecules now occupy 700 times the volume. This is close to 1000 (which is 10 x 10 x 10) so the molecules in a gas are about 10 diameters apart. In order for the sound to be transported to neighbouring molecules the molecules themselves have to travel about ten diameters before making contact. Hence sound travels in gas a lot slower than in solids, for example 330 metres per second in air as compared to 1450 metres per second in fresh water and 5000 metres per second in aluminium.

Light is an electromagnetic wave. When a beam of light enters into 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 a charged particle accelerating hence the atoms radiate, just like a radio transmitter causes electrons to oscillate in a vertical wire which then radiates radio waves. The wave radiating from the atoms has the effect of slowing down the light wave. As all materials have atoms, light travels at much the same speed in all solids and liquids, ranging from three quarters of the speed in vacuum for water to half the speed in vacuum 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.