Skip to main content
WalesPhysicsSyllabus dot point

How does stimulated emission and a population inversion produce coherent laser light?

Energy levels and photon emission, spontaneous and stimulated emission, population inversion and pumping, and the properties of laser light.

A focused answer to WJEC A-Level Physics Unit 2 lasers, covering atomic energy levels and photon emission, spontaneous and stimulated emission, population inversion and pumping, the optical cavity, and the properties of laser light.

Generated by Claude Opus 4.811 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

Have a quick question? Jump to the Q&A page

Jump to a section
  1. What this dot point is asking
  2. The answer
  3. Examples in context
  4. Try this

What this dot point is asking

WJEC wants you to relate photon emission to energy-level transitions, distinguish spontaneous from stimulated emission, explain population inversion and pumping, and state the properties of laser light. The laser is a beautiful demonstration that quantum ideas have real-world consequences, and the examiners expect you to link each property of the beam back to the physics of stimulated emission.

The answer

Energy levels and photon emission

Because the levels are discrete, only certain photon energies (and hence wavelengths) are emitted, which is why a laser transition gives a single sharp colour.

Spontaneous and stimulated emission

In spontaneous emission, an excited atom drops to a lower level at a random time, emitting a photon in a random direction. In stimulated emission, an incoming photon of the right energy causes an excited atom to emit a second photon that is identical to the first: same frequency, phase and direction. This doubling of identical photons is the amplification at the heart of a laser (Light Amplification by Stimulated Emission of Radiation).

Population inversion and pumping

A metastable state is one in which atoms linger longer than usual before decaying, which gives time for the population to build up. Mirrors at each end form an optical cavity so photons make many passes, building up the beam; one mirror is partly transmitting to let the beam out.

Properties of laser light

Laser light is intense, monochromatic (single wavelength), coherent (constant phase relationship) and collimated (a narrow, parallel beam). Each property traces back to stimulated emission producing identical photons in a cavity that selects one direction.

Examples in context

Example 1. Laser eye surgery. An excimer laser delivers ultraviolet pulses so monochromatic and collimated that they ablate a precise depth of corneal tissue with each pulse, reshaping the eye without heating the surrounding tissue. The coherence and tight collimation, both consequences of stimulated emission in a cavity, are what make this micrometre-scale precision possible.

Example 2. Fibre-optic broadband. A semiconductor laser switches on and off billions of times a second to send data down an optical fibre. Because the light is monochromatic, it suffers little dispersion and the pulses stay sharp over long distances, while the high intensity keeps the signal strong. The same stimulated-emission physics underpins the entire internet backbone.

Try this

Q1. An electron drops between two levels separated by 3.0×10−19 J3.0\times10^{-19}\,\text{J}. Find the frequency of the emitted photon. Take h=6.63×10−34 J sh = 6.63\times10^{-34}\,\text{J s}. [2 marks]

  • Cue. f=Eh=3.0×10−196.63×10−34≈4.5×1014 Hzf = \frac{E}{h} = \frac{3.0\times10^{-19}}{6.63\times10^{-34}} \approx 4.5\times10^{14}\,\text{Hz}.

Q2. State two properties of laser light and link one to stimulated emission. [2 marks]

  • Cue. Coherent and monochromatic; coherence arises because stimulated emission produces in-phase identical photons.

Exam-style practice questions

Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

WJEC 20194 marksA helium-neon laser emits red light of wavelength 633 nm633\,\text{nm}. Calculate the energy gap between the two laser levels in the neon atom, and state which type of emission produces the coherent beam. Take h=6.63×10−34 J sh = 6.63 \times 10^{-34}\,\text{J s} and c=3.0×108 m s−1c = 3.0 \times 10^{8}\,\text{m s}^{-1}.
Show worked answer →

Use E=hf=hc/λE = hf = hc/\lambda for the photon energy, which equals the energy gap between the levels.

E=hcλ=6.63×10−34×3.0×108633×10−9E = \dfrac{hc}{\lambda} = \dfrac{6.63 \times 10^{-34} \times 3.0 \times 10^{8}}{633 \times 10^{-9}}.

The numerator is 1.99×10−251.99 \times 10^{-25}, so E=1.99×10−25633×10−9=3.14×10−19 JE = \dfrac{1.99 \times 10^{-25}}{633 \times 10^{-9}} = 3.14 \times 10^{-19}\,\text{J} (about 1.96 eV1.96\,\text{eV}).

The coherent beam is produced by stimulated emission, in which an incident photon triggers an excited atom to emit a second identical photon, in phase and travelling in the same direction. Markers reward E=hc/λE = hc/\lambda, the value near 3.1×10−19 J3.1 \times 10^{-19}\,\text{J}, and naming stimulated emission.

WJEC 20223 marksExplain why a population inversion is necessary for laser action, and state how it is achieved.
Show worked answer →

For a beam to be amplified, stimulated emission must happen more often than absorption as light passes through the medium.

Stimulated emission requires atoms in the excited state, while absorption requires atoms in the lower state. Only when more atoms are in the excited (metastable) state than in the lower state, a population inversion, does emission outpace absorption and the light grow rather than fade.

The inversion is achieved by pumping: supplying energy (by an electrical discharge or a flash lamp) to lift atoms into the excited state faster than they decay. Markers reward the comparison of stimulated emission with absorption and naming pumping as the mechanism.

Related dot points

Sources & how we know this