In spontaneous emission, an excited atom decays without an incident triggering photon; emission directions and phases are not fixed together. In stimulated emission, a matching photon triggers a downward transition and an additional photon emerges with the same frequency, direction and phase. A laser amplifies this process using pumping, a population inversion and optical feedback. A metastable upper state helps maintain more atoms in the upper laser level than the lower one.
Fluorescence can convert one absorbed photon into several lower-energy photons, sometimes transferring energy internally. Each lower-energy photon has a longer wavelength than a photon carrying the full input energy. Laser light is coherent, approximately monochromatic and can concentrate power into a small area. Never direct beams toward eyes or reflective surfaces; beam calculations do not establish handling competence.
Example 1
An atom absorbs 5.00 eV and then emits photons of 2.00 and 3.00 eV, with no other loss. Check energy balance.
The emitted total is 2.00+3.00=5.00 eV, matching absorption. Each emitted photon has less energy and longer wavelength than the absorbed photon.
Example 2
A laser transition has upper population 600 and lower population 400. Is there inversion?
Yes. The upper-state population exceeds the lower-state population for the specified transition.
Watch out: The atom contributes transition energy as an additional photon matching the triggering photon’s frequency, phase and direction.