Question about the speed of light for you Science guys

Please don't insult me for my ignorance, but I'm thinking this may be an easy question to answer for those who know.

I get it that the speed of light slows down when it goes though something like water or glass.

typical example:
http://hyperphysics.phy-astr.gsu.edu/Hbase/geoopt/refr.html


Using glass as an example, when light exits the glass back into the air, does it resume at it's prior speed or is the light permanently slowed down from that point?

This is what's confusing me:


The illustration shows how the light resumes its path and I'm confused - does the light coming out back into the fast medium travel at the speed prior to entering the slow medium, however distorted, or does the speed of the light continue at a slower speed?

I'm confused because if the answer is that it resumes the original speed, I find that somewhat miraculous, but I find so many things about light miraculous, so I'm prepared for either answer.

Good question!

after leaving the diffracting medium.

Where's the gas pedal?

How does it speed back up???

...it shakes out that way, though the theory is much more complex.

but the short reason is there is no "it". A wave is a lot like the point of intersection between two scissor blades. We see it move, but there is no thing there. What we see moving is the medium in which the wave is present. and that's what changes speed...

There's the energy of the photon, and the "resistance" of the medium.

When the resistance decreases, the speed increases. The maximum speed, known as "c" occurs in a perfect vacuum.

I think you are confused because you're envisioning it like a person travelling along at a certain speed, losing energy and going slower, and the suddenly speeding up without any new energy input.

But the speed of a wave is entirely dependent on its medium. The speed has nothing to do with the wave itself. For example, a wave traveling in a rope depends on the tension of the rope, and nothing else. Because the wave isn't a real thing, it's a deflection in a thing (in this case, the rope).

light waves are distortions of electromagnetic fields. as such their speed depends on the "tension" (a metaphor here) of the EM field. In free space this tension is defined by two constants in maxwell's equations that have certain values for the vacuum. In dense media, the values for these constants are modified, thus the "tension" allowing propagation of the wave is changed.

That rawked. Thanks.

only the E (electric) and B(magnetic) fields do. and in certain media, E and B fields respond more quickly to deflections at nearby points in space than in a vacuum.

Dense media act to retard the responsiveness of E and B fields to deflection.

through. I read your link and believe they misspoke.

The speed of light in a medium is slower than in vacuum. In fact, it's 1/n times the speed in vacuum, where n is the refractive index, which is always >1. For diamond, n~2.4, so light travels 1/2.4 = 0.42 times as fast as in vacuum.

I would guess that it resumes its normal speed, as nothing is slowing its progress any more. But that's just my guess. I'll have to wait along with you for an expert to chime in.

it is just that when going through a dense medium it has to take a zig-zag course. It is like a constant 5mph walk through a forest, going around tress will slow your straight line progress to less than 5mph but you are still going 5 mph.

I was happy with staying classical and whatnot....but you had to get all quantum on my ass...

http://www.space.com/scienceastronomy/generalscience/lightstop_010119.html

Physicists say they have brought light particles to a screeching halt, then revved them up again so that they could continue their journey at a blistering 186,000 miles (299,330 kilometers) per second.


http://kn.theiet.org/magazine/issues/0909/slow-down-0909.cfm

Light, nature’s 300,000km/s speed demon, is being domesticated by a select group of physicists who are slowing it to just a few metres per second – the sort of speed a Sunday cyclist could easily achieve.

Fibre-optic communication is expected to be one of the first applications to benefit from the taming of light, which began in the United States of America just over a decade ago.

Think of a photon speeding a long through a vacuum at c. It enters a medium, which is mostly empty space where it still cruises along at c. But then it hits an atom and gets absorbed. Most photons don't like being in atoms, so it leaves the atom and continues along at c until it either leaves the medium or gets absorbed by another atom. At no time it actually travels less than c.

Now at a bulk, macroscopic level it appears the light is traveling less than c. Think of taking a trip down a highway with no traffic in a car that only goes exactly 60 mph. If you're traveling 600 miles the trip will take exactly ten hours. But if you stop for coffee a few times it's going to take longer than 10 hours, so if an outside observer is only looking at the time your trip takes, he's going to think your velocity was lower than 60 mph.

which is why physics is cool.

Waves are real, actual things, for example.

they're a mathematical construct that describes a process.

but perhaps it's better phrased that a wave is not an object, with boundaries.

A chair is an object. A sea of undulating chairs is a process, or behavior.

It's a breaking wave, as it has multiple z values for each x,y pair. It is not a single valued function of x,y, and time. As such, it isn't a physics wave.

And even with ocean waves, the water moves up and down only, until the water reaches a critical depth. The apparent propagation of the wave across the ocean surface is an illusion. It is in that sense I say that waves "don't exist".

You can't get any more classic than that.

There are breaking waves, there's a non-breaking wave, and waves that have already broken. All of which are physics waves.

All of these waves are moving, and that movement is toward shore. If you're mistakenly referring to the medium, no, that's not moving, excepting, as you allude to, in those breaking and broken waves. Both the medium and the waves exist.

I think you need to take a deep breath and take a step back for a second. You're overthinking things.

My point here is, was, and has been that there is no "thing" that is moving back and forth through the physical medium (unless we're talking about a longitudinal wave, which I'm not, and even then the medium doesn't move that far in the direction of propagation). The only thing moving is transverse to what the eye sees moving, and that thing is the medium. The movement in the longitudinal direction is an illusion. That is as close to "not existing" as anything can come, as far as I am concerned.

Conversation is done. I've repeated myself multiple times. You don't agree with my explication of the point. Fine. You want to call something that I identify as an epiphenomenon as a phenomenon. Fine. This has become unproductive and there isn't anything left to be said. Good day.

It's OK to be wrong. It's no big deal.

at least under any reasonable definition of "real" that I can concoct. Associated with waves are things like energy and momentum, for instance. I tend to think that anything that can impart momentum to objects (which waves assuredly can!) is pretty darn "real!"

But they're not "objects" in any usual sense of the word.