glass slab refraction again (only good thinkers allowed)?

Question

when light ray passes thru glass it is slowed and it bends . but wfen it re emerges out why does speed up to 3 x 10^8. where is it trigerred 2 such a speed . what pushes it

Answer 1

Nothing, actually. Speed of light is what light travel at, all the time. That speeds changes as a function of the medium. Think of it as some sort of spring. Entering the glass from the air compress the spring of the photon. When it leaves, it returns to its original shape and speed.

Another way of looking at it is that light gets absorbed and re-emitted by atoms along the way, with a short delay between absorbtion and re-emission. Transparent does not mean that photons go right through it. It means that light gets re-emitted at the same frequency it entered. Opaque objects re-emit at a different frequency that is a function of their temperature.

This is this short delay that explains the slower propagation.

Answer 2

When light strikes a black body it is absorbed because the photons of the light interact with the atoms (molecules) of the black object and are captured. When light passes through (transparent) glass the photons can interact with the atoms (molecules) of the glass but are transmitted almost unchanged (some light is lost). Perhaps the light slows down (depending on the density and refractive index of the glass) because it is like passing through a long series or turning styles. Perhaps the light slows down because there is a slight delay in being captured and released unchanged by the atoms (molecules) of the glass. Remember that leaded glass can contain perhaps 40% lead and still be transparent. It is transparent only because it allows photons to pass essentially unchanged.

Answer 3

For this, we need to simply assume that light is a wave.

Any electromagnetic wave has to keep moving, stopping only when absorbed by charged particles.

In glass, the light is not absorbed, so it travels through.

Also, light has a frequency. The frequency is intrinsic for any EM wave, and does not change wherever it travels, may it be glass, air, or whatever.

We also need to know the relationship between frequency and speed for any EM wave:

freq = speed / wavelength.

The speed here refers to the speed of light through a medium.

As light travels from air to glass, the wavelength gets shorter. Since frequency has to be preserved, the speed also has to be reduced.

The opposite thing happens when the light re-enters air. The frequency has to be maintained, and the wavelength increases, and goes back to what it was before entry into the glass. So the speed has to change again, this time incresing.

Answer 4

the question is interesting but the question "what pushes it" is based on false premises

a physicist would probably dicker with my explanation, but I think it might be helpful for you to consider it this way:

light travels at different speeds in different mediums

because light does not have real mass, and travels at the speed of light, it is a sometimes a mistake to think of it in Newtonian terms, and expect it to act like something at sublight speed with mass (like a bullet)

many aspects of the behavior of light will not be modeled by something like a bullet

light travels at different speed in different mediums

these speeds are the same always

when light enters a glass slab and moves at a slower speed in the glass (than it did in air or would in a vacuum) it has not lost momentum, it is not "slowed down" in the newtonian sense

when it returns to air, it does not have to be "pushed" to propogate at the speed of light in air

the momentum is the same in both media, even though the speed is different

Answer 5

Your question is a very good one. When thinking about light as a wave, wave mechanics gives us a reasonably satisfactory answer (as one poster hinted at above).

But the bigger problem is resolving this from a particle point of view. (light = photons). Turns out that photons ONLY travel at the speed of light, yet they magically and effectively seem to slow down in the aggregate as they travel through an optically denser medium. It turns out that this can be worked out as well, but to fully appreciate it you have to turn to quantum optics.

-T

Answer 6

light travels at various speeds through various substances, the speed of light through a vacuum is higher than that through glass, nothing pushes the light, it simply travels at a constant speed dependent on the substance it travels through

as the light emerges from the glass to (a vaccuum for example) it returns to the speed or light through a vaccuum