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.
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.
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.
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