---
title: "Stimulated Emission and How a Laser Works"
description: "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 separa"
canonical: https://lightmysky.com/learn/science/stimulated-emission-and-how-a-laser-works-mt_xgjD0mzEAV
source: https://lightmysky.com/learn/science/stimulated-emission-and-how-a-laser-works-mt_xgjD0mzEAV.md
retrieved: 2026-09-12
---

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# 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.

Subject: Science · Area: Waves, Light & Sound · Ages 21 to 22
Page: https://lightmysky.com/learn/science/stimulated-emission-and-how-a-laser-works-mt_xgjD0mzEAV

## Ready when they can

- 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

## Lesson: How a laser builds its beam

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.

**Example.** 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.

**Tip.** 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.

**Recap.** Invert the population, clone photons by stimulation, and recycle them through mirrors into a coherent beam.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Fourier Analysis and the Wave Packet](https://lightmysky.com/learn/science/fourier-analysis-and-the-wave-packet-mt_1ZaojWUZEw)
- [Separating the Schrödinger Equation for Hydrogen](https://lightmysky.com/learn/science/separating-the-schrodinger-equation-for-hydrogen-mt_Yk2qHB0_v2)

## Opens up

- [Ultrafast Spectroscopy and Watching a Reaction Happen](https://lightmysky.com/learn/science/ultrafast-spectroscopy-and-watching-a-reaction-happen-mt_wxQgetzzW5)