Question of the Week

Energy of Light

Sun, 4th Nov 2007

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Paul Tevendale, Woking asked:

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?


In vacuum and in air, to a greater extent, light is not really interacting with the medium it is in. Itís oscillating very fast like a very high frequency radio wave and travelling at its standard speed Ė 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 itís in a transparent medium canít excite the electrons, it isnít 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 itís 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 thatís been stored in the electrons is transferred back into the light field and it speeds up again.

Answered by Professor John Rarity.


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Is this all about refraction? If it is then i think the speed of light does not change and no extra energy is required, light is bent and travels further through the medium, glass. This gives the appearance of light slowing down.

Could be wrong, and I'm sure someone will come along with a much better answer., Wed, 31st Oct 2007

I'm not very sure, but I guess the maths ensures conservation of energy by making the mass (or effective mass) of the photons increase as they slow down...?

That, or its a trick question as the KE of the linear motion of the photon is actually tiny compared to its inherent photon-like energy    Arrrggghhh - I'm rusty on my basic physics!
techmind, Wed, 31st Oct 2007

When light passes through a substance (as opposed to empty space) it goes slower because, as it passes through the substance, it has to get the electrons excited as it goes past. This involves a bit of a lag, as the electrons start to oscillate and then release their energy. As the frequency of the photons does not change, there is no change of energy so nothing is violated.
Because of this interaction with the substance, there is some loss of energy (called absorption) sometimes a little, sometimes a lot, depending on the frequency and the energy gaps in the substance.
It's just fortuitous that light goes through water and glass without being absorbed  more than it is - useful too!

This is a situation where it's probably better to think in terms of waves rather than particles,
Photons have no mass, techmind.

Refraction is a consequence of this change of speed as light (and any other em wave) passes across a boundary at an angle. The frequency stays the same but the wavelength changes (because of the speed change). The resulting wavelength change gives you a change of direction when the 'beams' hits the surface at anything but a right angle lyner, Sat, 3rd Nov 2007

Could it not be that the light entering the glass medium is in fact still moving at the same speed through the glass, the glass particles merely increase the distance the light particles need to travel, thus appearing slower... phivos, Thu, 8th Nov 2007

The energy of light photons produced deep within the Sun takes years and years to reach the surface because it is constantly being absorbed and re radiated on its journey - this represents and extreme example of EM waves being slowed down by interaction with the medium they are traveling through. 'Between atoms' the light is traveling at c. lyner, Thu, 10th Jan 2008

I posted the answer on this website... variationz, Thu, 26th Nov 2009

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