On 613nm on the Absorption Spectrum

Regarding the absorption spectrum measuring 613nm, I was wondering, can it be said that of the two characterizations, emission and absorption, one is more accurate or more popularly used, or is one simply the direct inverse of the other? Nobody ever uses the emission spectrum unless you specifically state it. That’s just practical, since there are very few light sources, but everything in the universe absorbs. If you were talking about lasers, or radiation, then you would have a reason to talk about emission. For virtually any other thing, you would be talking about the absorption spectrum.

By the way, they are not at all inverse of each other. In fact, they are usually very similar. If you can get something to emit radiation, then it will usually emit at the same wavelengths that it absorbs. The reason for this is because both emission and absorption happen when an atom or molecule jumps between energy states. In the case of emission, the atom falls from a higher state to a lower one and the energy difference is emitted as a photon where the energy = h* the wavelength (h is Planck’s constant). For absorption, a photon with the energy equivalent to the gap between energy states in the atom will be absorbed and the atom moves from the lower state to the higher state. The quantum mechanics of the atom determines it’s energy states, but the gaps between them are the same in both directions (moving up to higher states or down to lower states). As for our eyes, there is obviously a big difference between the case where we see on pure wavelength (like in the case of a laser where you see the pure red color because that is the only wavelength emitted by the laser, and so when it reflects off anything it will stay that red color), and the case of white light bouncing off an object after having some wavelengths absorbed. Here’s a good experiment. Take a shiny patch of blue (like a tablecloth that is shiny and bounce a laser pointer off of it. It should still look red. See if you can bounce the laser off the shiny blue patch onto another object (say that is yellow), the light should still look red. Try the same thing with a flashlight. If you shine the flashlight directly onto the yellow thing it will look yellow. If you shine it onto the blue and then the blue light illuminates the yellow object, it will look different. – Baruch Sterman