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Lasers

A-Level laser extension: atomic transitions, stimulated emission, population inversion, optical feedback, beam properties, a helium–neon example and dedicated practice.

  • A-Level Laser Physics extension
  • 5 lessons
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Before you begin

A laser is not simply an intense lamp. Its operation links quantised transitions to population control, stimulated emission and resonator feedback. This is an extension beyond the H2 syllabus, built on the core quantum and waves topics. The lessons go from a single atomic transition to a complete device.

Be comfortable with: atomic transitions, line spectra and superposition.

Learning goals
  • Explain excitation, spontaneous emission and stimulated emission.
  • Explain metastable states, pumping, population inversion, threshold, optical feedback, output coupling and cavity modes.
  • Relate laser operation to coherence, linewidth, divergence, irradiance, photon quantities and a helium-neon example.

Lessons

Work through them in order.

  1. Atomic Transition Processes for LasersCompare absorption, spontaneous emission and stimulated emission using energy levels and photon energy.Beyond the syllabus
  2. How Lasers WorkConnect pumping, metastable levels, net gain, threshold, cavity modes and output coupling.Beyond the syllabus
  3. Properties of Laser LightDistinguish coherence, narrow spectral width, low divergence and high irradiance.Beyond the syllabus
  4. Helium–Neon LaserTrace discharge pumping, resonant energy transfer, inversion and the 632.8 nm red transition.Beyond the syllabus
  5. Laser Applications, Limits and SafetyMatch laser properties to sensing, communication, manufacturing and medical uses, within diffraction and safety limits.Beyond the syllabus

Practise and check

Topic reference

The organising model

Laser gain medium and optical cavityA pumped gain medium lies between a highly reflecting mirror and a partly transmitting output coupler, with matched photons amplified along the cavity axis.gain mediumhigh reflectoroutput couplerpump energyoutputfeedback selects and amplifies resonant modes travelling close to the cavity axislaser threshold: round-trip gain > round-trip loss
Scroll diagram horizontally to read all labels.
Pumping maintains a population inversion in the gain medium. Repeated passes through the resonator provide optical feedback; gain must exceed losses, and the partially transmitting mirror supplies the useful output beam.

Laser output builds only when pumping sustains inversion and the round-trip optical gain exceeds all cavity losses. Stimulated emission provides matched photons; the cavity provides repeated opportunities for amplification and selects the supported modes.

Interactive stimulated-emission explorer

Change the pump and cavity conditions, predict the dominant process and test whether the system reaches useful amplification.

Concept Explorer: Lasers & Stimulated Emission

Work through stimulated emission, metastable states, inversion, cavity feedback, and laser-light properties.

BetaReviewed enrichmentModern PhysicsBest for: A Level laser-enrichment revision with strong quantum links
  • Stimulated Emission
  • Population Inversion
  • Resonant Cavity Feedback
  • Laser vs Ordinary Light
Open the full interactive simulation on its own page

Use the standalone simulation page for the live controls, SVG scene, run modes, and scoring flow.

Open Interactive Simulation

The lesson stays lightweight and links out to the dedicated simulation page.

Revision checkpoints

Microscopic conditions
  • Absorption requires hf = E₂-E₁ and moves a particle to the upper state.
  • Spontaneous emission has random timing, phase and direction.
  • Stimulated emission produces a photon matching the stimulating photon in frequency, phase, direction and polarisation.
  • Net gain requires population inversion for the laser transition.
Device conditions
  • Pumping supplies energy and maintains inversion; it does not itself guarantee coherent output.
  • A long-lived upper state helps particles accumulate.
  • Resonator feedback makes repeated passes through the gain medium and selects modes.
  • Lasing begins only above threshold, when round-trip gain exceeds loss.
  • The output coupler must transmit some light while retaining enough feedback.