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So we know that as light moves from, say, a glass block to air, it speeds up and refracts, but if there is to be acceleration then surely there must be a resultant force to cause it? So where does that come from?

Secondly as the light enters the glass block it slows down, but where does the excess energy go?

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  • $\begingroup$ What makes you think the energy is related to speed in the medium? $\endgroup$ – Olin Lathrop Feb 19 '14 at 23:06
  • $\begingroup$ I think I was thinking of light as a particle... So I was thinking kinetic energy $\endgroup$ – user40951 Feb 19 '14 at 23:21
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    $\begingroup$ Photons have no mass. Their energy is not kinetic. It is a function only of their frequency. $\endgroup$ – Mike Dunlavey Feb 19 '14 at 23:42
  • $\begingroup$ Possible duplicates: physics.stackexchange.com/q/466/2451 , physics.stackexchange.com/q/2041/2451 and links therein. $\endgroup$ – Qmechanic Feb 20 '14 at 0:10
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There is certainly an interaction there between the optical medium and the photon. Actually, there are two photons in the interaction: the incoming one is absorbed by an electron in the material so that the latter fantastically fleetingly rises to an excited state. A fantastically short time later, another, outgoing, photon is emitted and the electron returns to its former state. In a transparent optical medium, the whole process is perfectly elastic (incoming photon energy is the same as that of the outgoing one). So the "deceleration" is actually an absorption, a delay and re-emission, at the speed $c$. The seemingly lower photon speed is actually not a lower speed at all; its simply that the photon made a stopover on the way!

You should be very wary of imagining photons as particles like billard balls, see my answer here, even though they certainly have definite particle properties (see my answer here), the foremost being that they are the discrete units of "communication", or more properly, interaction, between the second quantised electromagnetic field and the other quantum fields that make up the World. However, there is a kind of classical analogy which is fairly accurate in this instance.

Imagine a swarm of particles passing through a field of contraptions with perfectly elastic springs in them. Every now and again one of our little balls gets caught in one of these springy traps: the ball is slowed by the spring and undergoes simple harmonic motion in the direction of the incoming ball. Back and forth it goes and then suddenly the ball is let go again in the direction whence it came. As the ball gets caught in the trap, it exerts a force on the trap, and the trap exerts the opposite sense, equal magnitude force on the ball. So there's your force. Likewise, light being absorbed in an optical medium exerts a force on that medium: an impulse of $h/\lambda$ is transferred to the medium for every photon absorbed. As the ball oscillates back and forth in its spring, the spring exerts a sinusoidally varying with time force on the "chassis" binding all the spring traps together, and as the ball is accelerated and released by the spring, the impulse transferred to the field of springs by the ball when the latter is captured is now transferred back back to the ball, which continues on its way at its beginning speed. Notice that, when free, the balls speed is uninfluenced by the medium. If we measure the time of flight of balls, some will be "lucky" and pass through the medium with no delays and their speed will be measured as having not been changed at all; some balls will be captured once and some many times. The average speed includes the delays in the traps. It is this that begets the speed slowing factor, or what we know as refractive index.

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