Stimulated Emission and How a Laser Works
An excited atom hit by a photon of the right energy emits a second identical photon, and a population inversion plus a mirrored cavity turns that into a beam. Coherence, not brightness, is what separates a laser from a lamp.
What a learner can do afterwards
- Distinguishes spontaneous from stimulated emission and states what makes the emitted photon identical
- Explains why a population inversion is needed and how a three-level scheme reaches one
- Says what the cavity mirrors contribute to the output beam
1 · Read
An excited atom can drop on its own, firing a photon in a random direction at a random time: spontaneous emission. Or a passing photon can nudge it down, releasing a second photon that copies the first in frequency, direction, and phase: stimulated emission. Random versus clone is the whole difference.
Normally absorption wins, because more atoms sit in the ground state waiting to soak up photons. A laser needs a population inversion: more atoms excited than grounded, so emission beats absorption. A three level scheme reaches it by pumping atoms to a short lived band that drains fast into a long lived upper laser level.
Trap the photons between two mirrors and each pass doubles the clones through the excited gas. The mirrors leak a small fraction as the output beam. What leaves is monochromatic, directional, and coherent, unlike the spread out glow of a lamp.
Judge any laser design by three checks: a gain medium with a long lived excited level, a pump that inverts the population, and mirrors that recycle the photons. Missing any one leaves you with an ordinary lamp.
Invert the population, clone photons by stimulation, and recycle them through mirrors into a coherent beam.
2 · Watch
Take it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.