# Observing photons

Your replies to my question about being able to see a photon, from the side (answer, unanimous, “no”) have raised in me some additional questions. Would it reasonable to think that in consequence, the photon, from any angle but directionally “in front”, is a black body? I ask because I am accustomed to think of the photon as a “light” particle, when perhaps it might be more correct to think of it as a “black” particle. When an observer sees light from a photon, isn’t it true that in fact, what is actually being seen is the photon itself? When the photon enters the eye of the observer, what happens (3.1) to the observer? (3.2)to the photon? 3.1 The photon has no mass, so the observer/photon combination remains at original observer mass. However, the photon has energy, what impact on the observer done this energy input have?
3.2 Upon entering the observer, does the photon disappear? If not what else can it do?

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You might be misunderstanding the term "black body" here. A black body is something that perfectly absorbs and emits thermal radiation. So it can't possibly be used to describe a photon. Can you clarify what you meant? –  dbrane Jan 20 '11 at 22:27
I don't see your previous question, but rhodopsin does react when struck by a single photon. We wouldnt notice it though likely because of noise suppression by the brain. –  Gordon Feb 6 '11 at 19:01

In the case of an atom absorbing a photon, I suppose it is true that the apparent mass of the atom in its excited state would be infinitesimally higher than the ground-state atom's, by an amount $E_\gamma / c^2$, but that's such a trivial effect that nobody bothers to deal with it. To give you an idea of the scale, a typical atomic transition in the visible light range has an associated energy of a few eV, while the rest mass of a hydrogen atom is about 1,000,000,000 eV. You would never notice the effect (unless you're talking about high-energy gamma ray photons, in which case the result of absorbing a photon in the nucleus is usually to blast the nucleus apart after a very short time, often creating a bunch of new particles in the process).