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Whenever light enters from air to glass its speed decreases but when it cross the glass and again come in air its speed increases. What is the reason behind this property? What is slowing it and what is speeding it?

I have also read that it is true for all electromagnetic radiations.

Qmechanic
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Prashant Singh
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1 Answers1

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Light as a Particle

The photons in the beam of light are continuously being absorbed and re-emitted by the glass atoms (though this is also true in the other mediums light slows in). The level by which the light is slowed is dependent on how often this happens. As @garyp commented below this question, the delay also depends on how long the photon stays in the atom.

Light then speeds back up again as it is no longer absorbed and re-emitted as much (in air) or speeds all the way to the speed of light in a vacuum where there are no atoms to absorb and re-emit the photons.

Light as a Wave

Keeping in mind the particle explanation, first, the velocity of a wave in a solid is lower than the velocity of a wave in air. The second thing to keep in mind is what happens to the wave's phase as the photons get absorbed and re-emitted.

Depending on the natural frequency of the atom and the frequency of the wave, the emitted photons will change phase when compared to the other, unaffected photons. Therefore, it either falls backward or forward a bit. Since this happens every time the wave hits an atom and there are many atoms in even the smallest piece of material, this has the affect of slowing the wavefront as the wave advances through the substance.

An Analogy

It should also be added that light's speed doesn't slow down, rather, it is the interactions that slow it down. An analogy of this would be a car going 70 miles per hour on a highway with no other cars around, and then entering a congested area still going 70, but crashing into cars on the way through. (Obviously this isn't a perfect analogy, but it does give you an idea.)

More information is available here about this in terms of light as a particle. More information is available here in terms of light as a wave.

Hope this helps!

auden
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  • Why does it speed up again? – Prashant Singh Jul 14 '16 at 15:16
  • @PrashantSingh, I edited my answer to address that question. – auden Jul 14 '16 at 15:18
  • What if I consider light a wave? – Prashant Singh Jul 14 '16 at 15:21
  • I'm updating my answer to answer this question...one moment. – auden Jul 14 '16 at 15:25
  • @PrashantSingh, I have updated my answer to address the question of light as a wave. – auden Jul 14 '16 at 15:34
  • This is a very "high level" description, but I think it matches the spirit of the question. I'll point out that the "slowing" in the particle picture also depends on how long the photon dwells in the atom before being re-emitted. This is closely related to the phase delay in the wave picture. – garyp Jul 14 '16 at 19:51
  • @garyp, thanks. I've added that comment to the answer (plus an analogy I thought of). – auden Jul 14 '16 at 19:52
  • @garyp do you know if there is a chart that shows the ratios between time and distance for all the spectrums of light passing through glass? – Bill Alsept Jul 15 '16 at 18:01
  • @BillAlsept The ratio of distance to time for light in glass is just the speed of light in the glass, isn't it? $v=c/n(\lambda)$, so what you need is the refractive index as a function of wavelength. This website probably has more data than you can imagine. Select "glass" from the menu labeled "shelf", and then you have one or two :) types of glass to choose from under "book". (Odd names for the menus, but ...) – garyp Jul 15 '16 at 18:53