# Q & A: photons in and out of glass

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Most recent answer: 08/01/2016
Q:
I asked a question, where does light get it's "extra" energy when it leaves a block of glass, to resume travel at the speed of light. I also commented that at no point from the edge of the block of glass into the vacuum did the light accelerate. I was told to study quantum superposition, which I did but that doesn't seem to help for the following reason. Consider the last excited atom in the block of glass that emits a photon which "travels" and is absorbed for a fraction of a second by an atom outside the glass block which emits another photon. If both photons actually travel at the speed of light, why if we measure the speed of light "IN" the glass block is it slower??2nd question. Surely it is impossible to consider light travels in a straight line as opposed to a cone shape.Consider a spherical light source emitting either waves of light or streams of photons. Ten thousand metres from the tiny light source the distance between the waves, streams would increase. Like drawing a circle on a partially inflated balloon then blowing it up some more, as it gets bigger the Gap between the waves / streams would increase unless there were an infinite number of waves from the source. But if you imagine every emission of a photon traveling as an expanding cone shape, this would work. it would also explain why it could go through either of two slits to cause interference with itself. see part three
- Tim Harvey (age 56)
UK
A:

Since the frequency f doesn't change as the light goes in or out of the glass, the energy quantum hf doesn't change. Outside the glass the energy is pure photon energy, i.e. electromagnetic in a classical picture. Inside the glass the energy is partly in the form of electrons being partly in higher energy states than they were without the light.  (I'm not sure that this adds much to what you were already told.)

What happens to the momentum is a little more interesting. We've got an old answer that deals with this issue somewhat:  https://van.physics.illinois.edu/qa/listing.php?id=14665

Whether the wave is expressed as a quantum wave or a classical,  it spreads out over a huge range of angles. It's actually typically much broader than a cone. You can google "dipole radiation pattern" to get some images. So yes, this sort of wave picture is needed to explain two-slit interference. The photons are not little dots following trajectories.

Mike W.

(published on 08/01/2016)