If the ray of light (e.g. sunlight) is going through the atom, atom will absorb specific wavelength of light. Then, we see the unabsorbed wavelength part of light. For example, if atom absorbs red, we see blue (or green maybe?). But I learned that excited atom deexcites almost immediately. So doesn't it have to detect white light because the lights combine? Some people said that it is because of random direction of emission but I cannot understand. If it is so, when the distance between me and an atom is r, and the area of my eye is A, the probability of observing white light should be A / 4pi*r^2, isn't it?
From a dilute gas of sodium atoms say, you put a voltage
accross it to exite it till it glows. Then you analyze the spectrum composed of discrete lines.
Or light coming from the sun you notice some dark or missing
portions of the spectrum, that is atoms in the atmosphere that absorb this radiation. Fraunhofer lines.
That is innummerable examples with the aid or not of
spectrometers, of which there are more than one kind,
to see discrete lines of molecules or atoms.
Read spectroscopy.
You are correct. And if you plug in numbers, you find that this ratio is negligible for all but the most exotic cases.
Say, r = 1m and the surface of your eye is 1 cm^2, then you end up with a ratio of about 10^-5. In general, the approximations in whatever formulas you use or the errors from your equipment are larger than that.
There is a minor caveat in that during the emission, the polarization of the incident beam is preferred (classically speaking), so the emission is not completely isotropic, but that is not a huge effect, maybe an order of magnitude.
No, the probability of that happening is quite low. Also, you must be familiar with what happens when a photon strikes an atom. There are several process that undergoes depending on the initial conditions. It can be photoelectric effect, compton scattering or absorption as in your case. Although, we say that the emission is instant after the atom is in excited state, but there's no such thing as instantaneous in physics. The decay happens around to be 10^-8s, and there is this delay with respect to other wavelength you talked about. Also, the direction of instantaneous emission is random and it is not always that it will be emitted along the initial one. Further suggestions are welcomed.
Juan Weisz Yes, I knew about them.... But I didn't know why those absorbed lights are not observed by our eyes.
Ulf Lorenz Instead of our eyes, is it possible to detect them with machines? When I put light for 24 hours, and analyze spectrum for 24 hours, will there be a moment we can detect absorbed light?