Naked Science Forum

General Science => Question of the Week => Topic started by: thedoc on 30/10/2007 18:12:46

Title: QotW - 07.11.04 - Where does light obtain energy to speed up and slow down?
Post by: thedoc on 30/10/2007 18:12:46

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?
Asked by Paul Tevendale, Woking

                                          Read this Question from our Podcast (http://www.thenakedscientists.com/HTML/content/questions/question/1764/)

Title: Re: QotW - 07.11.04 - Where does light obtain energy to speed up and slow down?
Post by: thedoc on 30/10/2007 18:12:46

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.

Title: Re: QotW - 07.11.04 - Where does light obtain energy to speed up and slow down?
Post by: paul.fr on 31/10/2007 08:33:15

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.

Title: Re: QotW - 07.11.04 - Where does light obtain energy to speed up and slow down?
Post by: techmind on 31/10/2007 13:59:54

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!

Title: Re: QotW - 07.11.04 - Where does light obtain energy to speed up and slow down?
Post by: lyner on 03/11/2007 23:55:24

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

Title: Re: QotW - 07.11.04 - Where does light obtain energy to speed up and slow down?
Post by: phivos on 08/11/2007 09:03:55

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...

Title: Re: QotW - 07.11.04 - Where does light obtain energy to speed up and slow down?
Post by: lyner on 10/01/2008 13:13:37

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.

Title: Re: QotW - 07.11.04 - Where does light obtain energy to speed up and slow down?
Post by: variationz on 26/11/2009 04:48:40

I posted the answer on this website...
http://www.thenakedscientists.com/forum/index.php?topic=26911.0