# Help me settle a debate, Veeky Forums

Help me settle a debate, Veeky Forums

When it's said that light slows as it passes through some media, does that mean that the actual photons are moving at a lower speed, or is it that the time it takes for the photons to move from point A to point B is increased because their path is refracted and bent? I've always assumed it was the latter, because otherwise it seems like energy being created out of nothing when it "speeds back up" after re-entering a vacuum.

Am I right? I completely prepared to be told how stupid what I just said was.

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You are wrong.

geez stop being so insecure. Don't forget to ask your professor questions when you don't understand the subject, by the way.

The light waves literally move slower through the medium.
Suppose a beam of light that travels through a vacuum, goes through a piece of glass, and then travels out the other side. A wave would move at c through the vacuum, move with speed c/n through the glass where n is its refractive index, and will continue to move at c after it exists the glass.

I understand that there's a very real observable change in speed, but my question is about the actual mechanic behind that slowing. Isn't the refractive index of a material a measure of how much is refracts the path of the light? It's not a very important question because the observable change in time to get from one point to another is the only important bit, but I've always assumed that c really is a constant, and the slowing of light is just the complication of its path.

Otherwise, how does light speed back up to c when re-entering a vacuum without an external force accelerating it?

I think this is a legitimate question. Idk the answer, but I feel like it may be a â€˜complicated pathâ€™ kind of deal like u said

I think asapscience has a video on this
something about how the colour changes instead of the speed or something
All I know is the speed that photons move is always constant

Depends on the energy of the light, and the kind of material.

A not so dense material (like glass or organic matter) isn't gonna refract gamma rays. But in a medium of finite density and temperature, the low energy behavior of light quanta is modified. Collectively, low frequency (including visible light, often) electromagnetic waves are effected to such an extend that the group velocity of a wave (not to be confused with a single photon's velocity) can be lowered significantly below the speed of light in vacuum.

That doesn't tell you what photons are doing, since electric and magnetic fields aren't photons themselves. They are related in a complicated but tractable way.

When light travels through any medium, it forms a wave which has a lower speed than the speed of light in a vacuum through its interactions with charge carriers in the medium. This is similar to waves that form on a guitar string travelling at speeds less than the speed of sound of the guitar string. The actual photons themselves always travel at the speed of light.

How about when it's at normal incidence to a high index medium? The path is unchanged, but a delay is measured. This is because photons don't always travel at c. c is light speed in vacuum.

The material absorbs the photons and re-emits them, bouncing their path around

Light ALWAYS travels at c, the only time it doesn't seem to do so is when its path is elongated through materials

Both are correct
The light moves slower because its path changes
Light always moves at c because it has zero mass

This. It's the absorption and emittance that "slows" the light down. In between the particles light it still traveling at c

If this was true, wouldn't light be scattered rather than be refracted?

It's neither. Photons are absorbed and emitted again.

it's both scattered and refracted, but some light gets through in the direction it was headed

Whatever people try to say about individual photons, the fact of the mater is that you can't talk precisely about what any single photon does in an electrically charged many-body system.

Any single particle interpretation you have for the effect is going to have gaps in it.

This. It's way more useful and easier to think of them as waves imo.

Isn't the refractive index of a material a measure of how much is refracts the path of the light?
Refractive index is defined as n=c/v. It has EVERYTHING to do with the speed of light in the medium (the average speed of the wavefront from point a to b). The exact mechanism is beyond me. Learn quantum physics probably. c is a constant you mong.

first - its not due to bending because this also happens if light hits a glass brick straight on, not at an angle, and the difference in speed is measurable

the frequency=energy of the photons doesnt change either

whats left? Hint: lightspeed = frequency * wavelength
rest is leaft as excercise to the reader
this is used for example in immersion microscopy to see smaller details than light in air would allow

Pretty sure it's just how much the light is absorbed and reemitted within the material, correct me if that's wrong

ya all wrong fags, light does not move slower in any medium !!!

Think about fiber optics, does light move slower if the length of the fiber is 1000 km vs. 500 km. NO IT DOES NOT.

In some materials light bounces A LOT, so it seems like it might move slower through it EVEN THOUGH IT DOESN'T. THIS IS BASIC PHYSICS faggots THE SPEED OF LIGHT IS A FUCKING CONSTANT

How does gravity come into it? I know it bends the path of light but it's not interacting with any materials.

If you say fired a laser off a neutron star 'straight up' so it's traveling the exact opposite direction of gravity.

speed of light in a VACUUM is a fucking constant.

this

Okay so Photons are supposed to be aways traveling 'at C'. It's can take light to go thought a medium but the particles still traveling at C.

Okay so far. But since gravity can effect light what happens then?

Would the light escaping from a super-heavy-but-not-quite-a-black-hole-yet-star still be traveling a 'c'?

I've always assumed it was the latter
You've assumed correctly. Light does not speed up or slow down, it just takes longer paths through some materials than others.
Would the light escaping from a super-heavy-but-not-quite-a-black-hole-yet-star still be traveling a 'c'?
Yes.
en.wikipedia.org/wiki/Gravitational_redshift

Gravitational_redshift

Thanks, I did read over that but didn't find any mention if the speed is effect or not.

Reading up some more it does seem that protons are always going to at C in it's own frame of reference.

Still not sure what effect gravity has on the 'apparent' speed of light however. From what I could understand on the page on Shapiro Time Delay it seem that light does indeed (to an observer) seem to move slower when under the effects of gravity. If so how exactly is gravity causing the (apparent) slowing of light?

en.wikipedia.org/wiki/Shapiro_time_delay

If so how exactly is gravity causing the (apparent) slowing of light?

Light doesn't travel in a straight line though material, it gets scattered (elastically) on electrons, changing its direction by some angle.
So you will have straight line through vacuum, a bit zig-zag through glass and straight line after that again.
With some pythagoras you will find out that light has same speed, it just flew a bit longer through material than it's length is supposed to be

Veeky Forums doesn't know basic physics. sad as fuck. goodbye.

When a photons passes through a transparent medium it slows down. If it hits a solid object than it's reflected

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Refractive index is a measurement of the material's permittivity and permeability relative to that of a vacuum. Light is a traveling wave with coupled electrical and magnetic fields oscillating simultaneously with each other. These are real, measurable fields, not just theories or methods we use to better understand light and thus are subject to their appropriate physics. Matter of any kind interacts with fields at some level and thus components of light (being fields themselves) respond to the presence of matter. A system of equations describing the boundary conditions of light at a border needs to be written out to fully explain the solution, but in order for energy to be conserved while simultaneously meeting the boundary conditions an interface where light is approaching the medium with higher relative permitivity (aka a measurement of resisting an applied electrical field) must have a shorter wavelength while maintaining the same frequency. The solution does not require anything about the speed directly, and the changes observed in wave speed are just a result of the changes in wavelength primarily (v = dw/dk).

tl;dr - speed changes not due to refraction but due to how freely electric (and magnetic) fields are allowed to oscillate in them.

A wave is all there is to it.

so much cringe

Reference frames in a gravitational field are accelerating. Relativity applies to inertial reference frames. See physics.stackexchange.com/questions/33816/does-the-speed-of-light-vary-in-non-inertial-frames for a detailed answer