Is the color associated with a photon, a property of its wavelength, frequency or energy?
Nowadays, photons can be slowed down to near-rest, or so I hear. If a slowed-down stream of photons be incident on my eyes, how would my eyes interpret their color?
The speed of a photon has nothing to do with its frequency (or, consequently, its wavelength).
Energy = h(bar)*Frequency
Color is characterized by both wavelength and frequency, which are really two sides of the same coin. In reality, color does not exist. It is not an inherent property of light, it is created in the brain’s vision processing center.
Exactly. Since ordinarily, speed of light is constant, frequency <-> wavelength is a one-to-one function. But, if you slow down the photons, then a new relation is created.
So, when these slow photons hit your eyes, how would your brain assign them a color? IOW, what property of the photon does the brain respond to when assigning it a color?
You have me confused. So, does a photon slowed down to 0.5c have the same freq./wavelength?
Now that I think about it E = hv = hc/[symbol]l[/symbol]
Now is this c the speed of light in a vacuum, or is it the speed of light within a material? By slowing the photon down the energy would decrease, eventually to zero if you managed to bring the photon to rest. Since c can’t exceed the value of c in a vacuum, there’s no chance the contained energy can increase. Still if you bring the photon to rest the energy would be gone and the photon would cease to exist, which seems wrong.
Assuming I’m right, the wavelength would increase, becoming progressively more and more red shifted.
Q.E.D., any ideas?
A photon always travels at c. It’s a massless particle and therefore It can’t slow down. The apparent slowing of light in a medium is due to virtual interactions with the atoms of the medium – the atom-photon system forms a so called dressed state.
Slow photons can never hit your eyes, so the question is meaningless. Photons travel as fast as the medium through which they are traveling permits. When you see light, it’s traveling as fast as it can (nearly c) though vitreous humor.
What you’re really asking is “what is the effect on the frequency of light of slowing down photons?” The answer is “none.” The “slowing down” changes the wavelength; the frequency (and the energy) of the photons does not change. It might be expected from this that it would be perceived as the same color (as the color of light depends on which pigment it interacts with, which depends on photon energy), but the question is unanswerable.
Actually, the entire subject is meaningless, because photons DO NOT SLOW DOWN!!! EVER!!! The “slowing down light” experiments slow down signal pulses, not photons. Photons get absorbed and re-emitted, their energy, angular momentum, and other states get passed along, but they always, always, travel at c.
It occurs to me that in slowing the light down, the energy of the photon must remain constant. The frequency is going to remain constant, AFAIK, but the wavelength will change, becoming shorter.
I believe the cone cells in the retina respond to a photon’s energy, but I’m not sure of that. If that’s the case, then the perceived color should remain unchanged.
Q.E.D. is quite correct; if a photon is going to deliver information, it has to be absorbed, and if it’s absorbed, all it really does is tell us about its energy (ergo momentum as well) and angular momentum.
It’s associated with all three as for a photon all three are directly proportional to each other.
The photon itself isn’t really slowed down at all it always travels at c (see Ring’s post), in the experiment a light pulse is slowed in a very complex procedure.
So which is it, is color wavelength/frequency or does it not exist? I don’t see what is wrong with saying color exists. That’s like saying pain doesn’t exist, only momentum. Just because we perceive something in a certain way doesn’t make it non-existant. I think you’d quickly find yourself in solipsism if you invalidated all your senses by characterizing them as perceiving non-existent properties. And then we’d wonder how you know anything about anything at all, having done so. 
Color is [a function of] wavelength. I don’t see what’s wrong with that interpretation.
However, this is a good thread to ask my own sort of light question in.
We have two perspectives, the Relativistic Spaceship travelling quite fast (say, .5c) and the Stationary Observer who obviously isn’t really stationary but whatever. The RS is travelling in a line relative to the sun, and the SO is on earth (travelling mostly pretty slow). All photons leaving our ideal sun have energy e. Since the speed of light is the same for all observers, the RS sees a red-shift in the photons, meaning their wavelength has changed. But photon energy is dependent on wavelength, so the RS is going to measure a different amount of energy than the SO, right? But they’re the same photons. Why is this driving me crazy?
I should add that I’m not really sure what happens as to whether the color perceived would change; I’d have to think more about that. But it’s certainly true that it’s the energy that matters.
Gyan9, consider this.
Visible spectrum.
Not all frequencies are visible. As frequency decreases in the visible, we move from red to violet and drop out of visiblity into the Ultraviolet. In other words, if the light in question has a frequency in the visible range, you see it. If it doesn’t, you don’t. It is a simple tautology and no other considerations about the strange and wonderful thing we call light need trouble you.
It’s the other way around, UV has a shorter wavelength, and therefore a higher frequency than red light.
Oh, and erislover, it’s valid to say color doesn’t exist, just as it is valid to say flavor doesn’t exist. Or rather, it might be more accurate to say that these things exist only in the brain. Color isn’t an inherent property of light any more than flavor is an inherent property of molecules.
But colour is an inherent property of quarks and gluons and flavour is an inherent property of quarks (and neutrinos) 
erislover, yes the photons are redshifted and therefore have a lower energy in the reference frame of the spaceship than they do in the reference frame of the stationery observer (the conservation of energy still applies though).
Is mass not an inherent property of non-bosonic matter, then, and instead only exists in balances? Anyway, GD territory, I guess.
For the color to change perceptibly, the receptors in the eye itself would have to be part of the “slow light” system. Otherwise you’d just be detecting regular old speedy green, or whatever, photons that had previously been part of an optical molasses. However, as soon as you cool down the eyeball, you be messing with the conformation of Rhodopsin, as well as the action spectra of the associated pigments. As a practical matter, cooling your eyeball down to the temperature of a Bose Einstein condensate would likely cause it to crack open :eek:
Yeah, what I don’t get is that I always heard that you will ALWAYS report photons as traveling at c relative to you, period. But if that’s true, how does the Doppler effect happen?