Light-Matter Interaction and Photons

The photon concept and the quantum treatment of how light is absorbed and emitted by atoms underlie the discrete exchange of energy between light and matter.

Definition

The description of light as quanta of energy and momentum, photons, and of the quantized processes by which atoms and molecules absorb and emit them, governed by energy conservation and quantum transition rules.

Scope

This topic covers the quantum nature of light as discrete photons and the fundamental interactions of light with matter. It includes the historical evidence for photons from the photoelectric effect and the Compton effect, the energy and momentum of a photon, the quantum-mechanical treatment of absorption, spontaneous emission, and stimulated emission, selection rules, the role of the density of states and cavity quantum electrodynamics in modifying emission, and single-photon detection. It connects the particle aspect of light to atomic energy levels and provides the foundation for laser gain and for single-photon technologies.

Core questions

• What experimental evidence shows that light is quantized into photons?
• How much energy and momentum does a photon carry?
• How do atoms absorb and emit photons in transitions between energy levels?
• How can the rate of spontaneous emission be modified?

Key concepts

• photon energy and momentum
• photoelectric effect
• Compton effect
• absorption and emission
• selection rules
• spontaneous emission rate
• cavity quantum electrodynamics
• single-photon detection

Key theories

The photon and the photoelectric effect

Einstein proposed in 1905 that light energy comes in quanta of energy proportional to frequency, explaining the photoelectric effect; the Compton effect later confirmed that photons also carry momentum.

Quantum theory of absorption and emission

Transitions between atomic energy levels absorb or emit photons of matching energy; spontaneous emission, stimulated emission, and absorption are treated quantum-mechanically, with emission rates depending on the electromagnetic environment.

Clinical relevance

The photon picture underlies the quantitative dosimetry of light in phototherapy and photodynamic therapy, the operation of single-photon detectors in fluorescence lifetime imaging and positron emission tomography scintillation, and the interpretation of how light deposits energy in tissue.

History

Planck's 1900 quantization of energy and Einstein's 1905 light-quantum hypothesis introduced discreteness into radiation, with the photoelectric effect work earning Einstein the 1921 Nobel Prize. Compton's 1923 scattering experiment confirmed photon momentum, and Dirac's quantization of the field in 1927 gave the modern theory of light-matter interaction.

Key figures

• Albert Einstein
• Max Planck
• Arthur Compton
• Paul Dirac

Seminal works

• loudon2000
• einstein1905

Frequently asked questions

Is light a wave or a particle?

Light exhibits both wave and particle behaviour; it propagates and interferes as a wave but exchanges energy and momentum with matter in discrete quanta called photons, a complementarity captured by quantum theory.

What is the energy of a single photon?

A photon's energy is Planck's constant times its frequency, so higher-frequency, shorter-wavelength light such as ultraviolet carries more energy per photon than lower-frequency light such as infrared.