Science Questions

Energy of Light

If the speed of light is slower in glass than in air, where does the energy come from to speed it up as it exits the glass into air? Why does this not violate the law of conservation of energy? Paul Tevendale, Woking

In vacuum and in air, to a greater extent, light is not really interacting with the medium it is in. It’s oscillating very fast like a very high frequency radio wave and travelling at its standard speed – the speed of light as we know it. When it enters the glass, the glass contains lots and lots of atoms. Around the atoms are electron clouds. Now, the light when it’s in a transparent medium can’t excite the electrons, it isn’t absorbed, but the electrons do like to try and follow the oscillations of the field. As the electrons are trying to follow the oscillations of the field it means that some of the energy of the light is stored in those electrons. And that whole process, in effect, slows the whole light field down. So the light in the glass is a combination of the electromagnetic wave and the polarisation, as it’s called, of the electrons which are travelling together through the glass. This oscillation of the electrons actually gives a rise to this dielectric constant and refractive index. Of course the refractive index is the ratio by which the light is slowed as it travels through the solid material. When the light reaches the end of the solid material, it goes back into air or vacuum and there are no more electrons so the energy that’s been stored in the electrons is transferred back into the light field and it speeds up again.

Answered by Professor John Rarity.

November 2007