Atoms

Yes, a single atom can send out light. Each atom (or molecule) has a certain configuration of electrons and thus can send out a specific configuration of light. I'm not sure if there are any instruments that are sensitive enough to pick up single photons, though.Atoms have colour in exactly the same way a painting do.

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Just to be sure I understand correctly, when you say that atoms "have" colour, do you simply mean that the material properties which cause a given element to reflect the light that my eyes interpret as a certain colour are contained within each of the element's individual atoms?

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Or are you saying that each of its individual atoms actually reflects light in exactly the same way as does the whole? That if we had an ordinary photographic camera, with an atomic-level magnification, the individual atoms would actually show up on the photographs as having colour? If so, would their colour be the same as that of the macroscopic whole (assuming, for the sake of argument, 100% purity of the element in the overall grouping)?

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What gives an object a certain colour isn't reflection. It's absorption of certain frequencies of sunlight (or other energy, electrical lamps work) and the emitting of those towards any observer. This works ONLY on an atomic or molecular level, and if you have more of them, it just adds up the intensity.You are correct that reflection, and other similar phenomena like the usage of x-rays to examine crystalline structures, often (but not always) depend on more than one atom, but those are not what determines colour.

No, we claim exactly that; science can often explains the how and what but hardly ever the why.

Single atoms would display colour in the same way that they do when there are millions other around them. Most theoretical models of how light is sent out does actually model just a single atom, because that's all that is needed for the principles to hold.We see colour because stuff in our eyes recieves photons. If a photon that has the wavelenght of, say, green enters the eye, it's detected as green light. If it's just a single one, it's basically filtered away as random noice, but it's still detected.A single photon does not show up as a large bright green blob in your brain, because then you'd be blinded constantly. You are not conciously aware of the single photons you see, if that's what you are wondering. You'd need to magnify them quite a bit first.The wavelenght has nothing to do with the size of the atom. A normal radio-antenna can pick up wavelenghts far longer than it's own lenght. The two lenghts are not at all related.

A single atom isolated somehow will, if you exite it via sunlight or electricity or heat radiation or whatever, send out photons of it's own, yes. It is not in any way dependant on surrounding photons to do so.What I ment with magnify is that if we have a machine that picks up a single photon, and sends out 10 photons of the exact same type, our brain can see that as a coloured dot.When we say atoms have a colour, we mean that the electrons of that atom sends out light with a certain frequency that is interpreted in the brain as colour. The other parts of the atom, like neutrons, does not send out light and are as such invisible. We still consider the light coming from the electrons as coming from the atom as a whole.Wavelenght and frequency are directly related by a simple formula, in the case of light it's Frequency = Speed of light divided by Wavelenght. So each frequency have one specific wavelenght, since the speed of light is constant.The wavelenght of an EM-wave is not related to how large the wave is, or how much space it takes up. It is only related to the shifting of the electrical (E) and magnetical (M) fields. A ray is completely straight.The frequency also depends only on the EM properties of the ray, and describes how many 'peaks' (in field strenght) that passes a certain point per second.None of these has anything at all to do with size or largeness; all light-waves are exactly the same size. It's just the behaviour of the energy it carries that changes.

When it comes to the size of light, there isn't really a clear answer, because;A photon of light (particle) is modeled to be a point that has no volume, but travels at the speed of light. A source of light sends out a specific limited amount of these, but they would in total have a volume of zero.A wave of light (wave) is modeled as a sphere that is filled up with a changing electro-magnetical field. It has a volume that depends on how long time it was since the source was 'turned on'.You're going to spend a lot of time getting confused over this; I still am even if I know how the models work. I can't tell you a way to turn this into something that makes sense, because they don't make sense to me. I know they work because I know how to apply them to experiments with electronics, but that's about it...Wavelenght does not have anything to do with the physical size of the wave. The diagrams where you see a sinus-wave that represents a ray of light is missleading because it doesn't show location or displacement, but, and this is important, the strenght of the electrical field. There should be another sinus-wave that goes in and out of the page that shows the strenght of the magnetical field.This message has been edited by Melchior, 12-03-2004 02:46 PM