Science Questions

Why does glass allow light through, and how does it speed up again?

Sun, 10th Apr 2011

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Tim Harvey asked:

Why does glass allow light to pass through it?


From Mr Bahnu




If light travels through a glass block, and then enters into air or a vacuum it's speed will increase . Where does the engery come from to increase the speed?




Dave -   The easiest way to explain the reason why glass lets light through it is that it just can't absorb the light.  Itís a smooth surface, so light can get in to it, and it just doesnít happen to have any electrons which are able to absorb light of visible frequencies in there, so it can't absorb, so light carries on going as if nothing had happened to it.  The only effect that it does have is that the electrons in the glass do slow the light down a bit.  It doesnít take any energy away from it.  All it does is slow it down because essentially itís moving through a denser medium, a medium with more inertia to it.  So once it leaves the glass again, it moves into the air or a vacuum which is again a less dense medium, at which point, the light has still got all the energy it had before and it can carry on at its original speed.


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Tim Harvey asked the Naked Scientists: So, if light travels through a glass block, and then enters into air or a vacum it's speed will increase . Yes? Where does the enegry come from to increase the speed? Tim What do you think? Tim Harvey , Sun, 6th Feb 2011

Hi, Tim,  Welcome.

Sometimes it takes our experts a while to respond, so I shall chuck in a non-scientist's view to be getting on with.

As I understand it, a photon always travels at "c". However, when it is passing through a medium such as glass it is absorbed and re-emitted by atoms in that medium.  It is this process that slows it down.  When it emerges it continues at "c", as it had been doing between successive hold-ups in the denser medium. Bill S, Sun, 6th Feb 2011

The energy as you discuss will be down converted when a photon pass through another medium than a vacuum. How much is a question about what the material is, but you need to remember that it's not the 'original photon' coming out from that glass, but a photon created through all those interactions with 'matter atoms' and electrons jumping valence bands, and sending out a 'new photon' again.

Photons also change energy in space due to red-shift and expansion. But the question there is how? And if they really lose what 'energy quanta' they were seen as existing of 'originally'. There is a difference in a 'interaction' like with electrons in a atom, and a 'expansion'. In a 'expansion' there is no real 'interaction' taking place as the photon, if so, needs to be annihilated, to then be resurrected. The only way to define how a photon act with 'expansion' is in fact to stop looking at as a 'energy quanta' and instead define it as a wave, as waves can be 'stretched out' but photons can't.

the lights speed huh :)
That's a 'constant', meaning that it's similar to you finding one and one being two. No reason for light always go at 'c', more than it does, in all experiment we've been able to define. And this makes it a indisputable truth, even though it's a popular question. yor_on, Sun, 6th Feb 2011

Tim, I am not sure of the deep mathematical answer - but the general handwaving explanation is as follows.  When light is travelling through a medium (ie not through the vacuum) it is thought of as being absorbed and re-emitted by the atoms/electrons that make up that material.  The speed that light travels between these absorption/emission operations is 3*10^8m/s (ie speed of light in a vacuum) - but the practical measure of speed through a material (and thus the refractive index) also adds on the time spent being absorbed and re-emitted.  So when the light leaves the medium it continues to go the same speed as it was BETWEEN the absorption/emission operations ie 3*10^8 m/s. 

So it has never stopped going at 3*10^8 m/s - it has just spent some time doing something else.   

nb In certain materials (exotic condensates) the time spent "doing something else" is so huge that light is "slowed" to a walking pace - and researchers believe that a few more tweaks will allow manufacture of material that once light goes in it will be slowed/stopped completely. imatfaal, Mon, 7th Feb 2011

Allow me to add a little to this explanation.  Light travelling through a clear material like glass has not got sufficient energy to seriously disturb the electrons and change their orbits in any way (that is why glass is transparent) so the electromagnetic waves just distort the atoms which bounce back like superballs with no loss of energy Except for the odd few that happen to hit any opaque impurities in the material and get absorbed  (nothing's perfect) this bouncing does not disturb the direction the photon is moving (if it does the glass is opalescent) but it does delay it a bit and that is why light travels slower through matter. Soul Surfer, Tue, 8th Feb 2011

Light doesn't travel through anything. Light is the observation of the vibration of electrons. Electrons at the source vibrate at frequencies that our eyes can react to. Those electrons influence electrons in front of them (the gas or near-vacuum in a light bulb. Those vibrate electrons in the glass, which in turn vibrate electrons in the air. If passing through more glass, then they vibrate electrons in the glass. The glass appears to 'pass' light because it passes those vibrations properly. Changing its shape alters the direction that the vibrations are transmitted, which is how lenses are made. The vibrations again carry on to electrons in the air, and then through the various parts of our eye (such as the lens). No matter is being transmitted. Only vibrations are transmitted. Tom, Wed, 13th May 2015

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