Physics Van 3-site Navigational Menu

Physics Van Navigational Menu

Q & A: emission and absorption lines

Learn more physics!

Most recent answer: 01/10/2012
Q:
why does the dark lines of absorption spectrum have the same wavelenghts as bright lines of emission spectra?
- medha (age 17)
annandale, va, usa
A:
Here's where those lines come from. When an atom (for example) interacts with light by far the easiest way for them to trade energy is for a single quantum unit of light (a photon) to be either absorbed or emitted. For that to happen the atom must have two states with definite energies that differ by just the amount of energy in one of those photons. That's needed so that energy can be conserved in the process of the atom changing states while emitting or absorbing the photon.

So if the atom has two states whose energy difference just match the energy of a, say, red photon, atoms that happen to be in the lower-energy stat can absorb photons of that color. Atoms that happen to be in the higher energy state can emit photons of that color. (Unless the atoms are quite hot, or somehow have had their energy raised in a more specific way, most will sit in the lower energy state.)

(Experts will know that there are also some other conditions on the states require for these processes to occur.)

Mike W.

(published on 01/10/2012)

Follow-up on this answer.