What causes light to slow down when it passes through glass?

Question

I'd like an actual physical reason why it slows.

Answer 1

Light "slowing down" when it passes through an optical medium like glass is an approximation, but it is a very accurate approximation.

In the context of classical electromagnetic theory, the answer is very clear: the atoms in the glass can be slightly "polarized" (acquire an induced dipole moment) by an electric field, and this reacts back on the electric field. The resulting change in the electric field is ascribed to an electrical permittivity that differs from a vacuum. Since the speed of light depends on the permittivity, the result is a change in the speed of light in the medium.

In a quantum context, the answer is not so clean. A photon of the light, in its probabilistic meanderings through the material, has a chance of being absorbed and re-radiated by any of a very large number of atoms near its trajectory. Each absorption and re-radiation delays the photon a little bit. The combined effect of the many possible absorptions and re-radiations in the wavefunction combine to a very high probability that the photon will go straight through the glass, but at a lower average speed than the speed of light in a vaccum.

Answer 2

Light doesn't actually get slowed down. Its average speed within a material, such as glass, is less than its speed in a vacuum, but not really because the light slows down. The light gets "delayed" by atoms. This occurs through absorbtion and reemission with high-probability that the "released" light will travel in the same direction as the original photon that was absorbed.

The same process happens again. As a result of these tiny delays, the light takes longer to pass through one meter of the material than through one meter of empty space.

If you are asking this question, then you might find Cerenkov radiation interesting. It is what you see when an object travels faster than the speed of light in a medium. It is a *beautiful* actinic blue glow. I've seen it looking down into the core of a Triga reactor through 30 feet of distilled water.

Aloha

Answer 3

The speed of light in a medium depends upon the permittivity (e) and the permiability (m) of the medium.

e defines the ability of the medium to hold and electrical charge and m it's degree of magnetisation in response to an applied magnetic field.

Maxwell took existing expressions describing the behaviour of static electric and magnetic fields and demonstrated the existance of electromagnetic waves and their speed. If you decide to look at Maxwell's equations, you will need to have a sound grip on vector maths and calculus (this is not for the mathematically challenged!).

One of Maxwell's results was that the speed of light =
1/(e * m)^(1/2)

The refractive index of a material is given by the ratio of the speed of light in free space to the speed of light in the material.
Knowing the values for e and m in free space and in the material allows you to calculate both the speed of light in the material and it's refractive index.

To truly understand what is going on requires a good understanding of Maxwell, however.............

Imagine a beam of light with a certain power, that is transfering a given amount of energy per second. In a cubic meter of space, energy is stored in the magnetic field and the electric field. The amount of enrgy stored in the cubic meter or space by the electric field is determined by e and in the magnetic field by m. If these values change then the amount of enrgy stored in the cubic meter of space will change. To maintain the power in the beam of light constant, the speed at which the meter cubes of energy travel must also change, hence the change in the speed of light.

Answer 4

Refraction is commonly explained in terms of the wave theory of light and is based on the fact that light travels with greater velocity in some media than it does in others. When, for example, a ray of light traveling through air strikes the surface of a piece of glass at an oblique angle, one side of the wave front enters the glass before the other and is retarded (since light travels more slowly in glass than in air), while the other side continues to move at its original speed until it too reaches the glass. As a result, the ray bends inside the glass, i.e., the refracted ray lies in a direction closer to the normal (the perpendicular to the boundary of the media) than does the incident ray. A light ray entering a different medium is called the incident ray; after bending, the ray is called the refracted ray. The speed at which a given transparent medium transmits light waves is related to its optical density (not to be confused with mass or weight density density, ratio of the mass of a substance to its volume, expressed, for example, in units of grams per cubic centimeter or pounds per cubic foot. The density of a pure substance varies little from sample to sample and is often considered a characteristic property of the substance).
In general, a ray is refracted toward the normal when it passes into a denser medium and away from the normal when it passes into a less dense medium.

Refraction index of glass is 1.5. (thats the ratio of speed of light in vaccum to the speed of light in glass).

So................. now u know!

Rock on!

Answer 5

It is because it must be "propagated" through the material. There is an interaction between light and the material it is passing through, and this interaction slows it down.

See the supplied link for more information

Answer 6

It's called 'refraction" of light.It disperse light ...that's why you see the 7 colours of rainbow...God bless!