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Name: George
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We know that acceleration can be caused only by a force. But, when light passes through a glass slab, its speed decreases. And again as it enters the air, it regains its original speed. Which force caused light to increase its speed again after passing through the glass slab?

Dear George,

A good question! The answer in detail is complicated, but the basic idea is simple.

Light, as Einstein taught us in 1905, always travels at the same speed, c, as measured by any observer (even by two observers travelling at high speed relative to one another). However, the effective light wave front traversing a piece of glass which has an index of refraction of, say, 1.5 travels at a speed of c/1.5.

The resolution to this seeming paradox is that light traversing a piece of glass (or any other transparent medium) gets absorbed and then quickly re-emitted by the molecules in the medium, Each photon (piece of light) travels at speed c between atoms, but then is stopped by a molecule which emits another a short time later travelling in the same direction. It is obvious that the average speed will be less if the light stops once in a while. But the detailed calculation of the times and why interference effects keep the light going in the same direction is not at all trivial and is only discussed in advanced graduate courses on optics.

I should add that whenever a photon is absorbed, a force is transmitted to the molecule and thence to the lattice. When a photon is emitted in the same direction, a force in the opposite direction is transmitted to the molecule. These forces are extremely tiny and since they occur in pairs they cancel out on the average.

Note that the light speed of the wave leaving the medium is no more than the light speed inside the medium between molecules.

Best, Dick Plano, Professor of Physics emeritus, Rutgers University


Light is one of those effects which acts sometimes like matter, (such as a ball being thrown), and other times like energy (such as an ocean wave).

In the case of moving through glass, light is acting more like a wave. Wave that travel across shallower waters will slow down, yet grow in height. Thus, thier total energy remains the same. Upon leaving the plate of glass, the light can resume its original velocity.

Ryan Belscamper

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