[EvaH (OP)]

Jeff wants to know:

The speed of light is always given "in a vacuum".  Why that qualification?  What causes light to move at a slower speed when passing through matter?  Don't the photons mostly pass through empty space between nuclei and electron rings?  Is light actually slower, or just bouncing around until it's absorbed?

Can you help?

[guest45734]

The speed of light passing through water is significantly slower than in a vacuum. Cherenkov radiation might interest you, its a bit like a sonic boom caused by particles travelling faster than light in water

https://en.wikipedia.org/wiki/Cherenkov_radiation

[Janus]

The "in a vacuum" qualification is important to relativity.   All formulas and equations relate to c.  c is the speed at which light will travel through a vacuum.  The fact that light travels slower through water will not result in the effects of Relativity being any different, because even in water c remains unchanged even if the actual speed of the light through the water has changed.

Light isn't the issue at all. it is the speed c.  Light in a vacuum happens to travel at this speed, so it makes a convenient reference.

[geordief]

The "in a vacuum" qualification is important to relativity.   All formulas and equations relate to c.  c is the speed at which light will travel through a vacuum.  The fact that light travels slower through water will not result in the effects of Relativity being any different, because even in water c remains unchanged even if the actual speed of the light through the water has changed.

Light isn't the issue at all. it is the speed c.  Light in a vacuum happens to travel at this speed, so it makes a convenient reference.

Is it the case that light does travel at c in water? Is it a case of stop-start with the stops occurring every time the light wave interacts with the medium and the starts being periods where it continues at the speed of c?

It is the periods of interaction that appear to slow the light  down whereas it actually travels some of the time at zero speed and the rest of the time at c?

[guest45734]

The heading of the post is "Does light move at different speeds through different materials" the answer is yes (on average). However as pointed out by geordief, the movement of the photon is most likely slowed by interaction with molecules whereby a photon is absorbed by an atom raising it to a higher energy level then being released allowing the atom to go back to its original energy level. Not all the light entering a "material" will pass through, some of it will no doubt be absorbed. But as pointed out by Janus the speed of light in a vacuum is c. A photon does not slow down unless it interacts with something ie a material, but then between atoms in free space c is c .


Space is not completely empty it is full of virtual particles and approx 1 proton and 1 electron per square centimetre ie Hydrogen. Can a low density of electrons or a mixture of quarks slow light down in free space ?

[alancalverd]

It is not helpful to use the photon model when considering the speed of light, since there is nothing in the model that predicts c or v_m in a medium. Maxwell's wave equations derive the speed of light in terms of the permeability μ and permittivity ε of a medium, both of which can in principle be measured independently. The result is that v_m < c for all media including any less-than-perfect vacuum.

[Bored chemist]

It is not helpful to use the photon model when considering the speed of light,

You might want to discuss that with anyone up to, and including, Fizeau who measured it, and who only had a particulate model or an uncertain model (i.e. "is it a particle or a wave? nobody knows") to work with.

[alancalverd]

The problem with the particle model is that it doesn't give you any indication of the propagation speed, until you get to accept a whole bunch of counterintuitive relativistic quantum concepts which are circularly based on the constancy of c, whereas Maxwell's wave equations derive c from measurements of ε₀ and μ₀ that you can make in any electrical engineering laboratory.

And explaining why v_m < c for all m involves a whole lot of hypothetical assumptions about slowing or stopping photons (see e.g. dead cat, above) if you start with photons, whereas Maxwell just involves measuring ε and μ for a real material, in the same lab.

Fact is that electromagnetic radiation is electromagnetic radiation, and we have two quite different mathematical models to predict its behaviour.

[jeffreyH]

If you pass light through some supercooled materials you could easily outpace it with conventional transport. So what?  Read what has been written above about the Maxwell equations. This is a unique learning opportunity.

[jeffreyH]

Anyone interested in history can read about the Fitzeau experiment
 here

[Bill S]

…..A photon does not slow down unless it interacts with something ie a material …..

If a photon reacts with something, doesn’t it cease being a photon?

[geordief]

…..A photon does not slow down unless it interacts with something ie a material …..

If a photon reacts with something, doesn’t it cease being a photon?

I thought a photon only became a photon when the light wave interacted with something.

Or is the wave a classical description and the photon a quantum description ? (totally confused)

[Colin2B]

I thought a photon only became a photon when the light wave interacted with something.

Or is the wave a classical description and the photon a quantum description ? (totally confused)

You have to remember that a photon is a measure of energy. When light is ejected from an atom it has a specfic energy due to the change of energy level of the electron in the atom; when it is absorbed at the detector it has to stimulate a similar but opposite electron energy shift and that causes an effect which is measured. Light with the ‘wrong’ energy is not detected, that’s why the term photon is linked to the measurement points.

[yor_on]

they are a duality geordie. you can set up a experiment shooting 'single photons' or you can use a flashlight. It depends on your setup what you will measure.

[Bill S]

However as pointed out by geordief, the movement of the photon is most likely slowed by interaction with molecules whereby a photon is absorbed by an atom raising it to a higher energy level then being released allowing the atom to go back to its original energy level.

Following this reasoning, I’m considering a single photon passing through a block of glass; for simplicity, I have the glass surrounded by a vacuum.  It approaches the glass at c.  Once it enters the glass, it is absorbed by an atom, and then re-emitted.  This process is repeated as it continues to pass through the glass.  This means that it is not a single photon that travels through the glass; it is a succession of new photons, created at each new emission.  Any travelling done by the photon within the glass is at c; the apparent slowing results from a succession of minute instants during which the photon does not exist. 

One problem I have with this is that the refractive index of glass varies continuously, rather than abruptly.  If the above reasoning were correct, this would not be the case. Atoms have discrete energy states, so the absorption and emission spectra would be discrete. The theory doesn’t fit the facts.