Britannica Sources used in this answer

How are electromagnetic waves described differently in classical physics and quantum theory?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Diagram of a tree leaf illustrating photosynthesis, with arrows showing sunlight, water, and carbon dioxide entering the leaf and sugars and oxygen being produced. A person is shown beside the tree to demonstrate the exchange of oxygen and carbon dioxide between plants and animals. — electromagnetic radiation Diagram of photosynthesis showing how water, light, and carbon dioxide are absorbed by a plant to produce oxygen, sugars, and more carbon dioxide.

In classical physics and quantum theory, electromagnetic radiation is described through different conceptual frameworks, though both are essential to modern science.

Classical Physics

In classical theory, electromagnetic radiation is viewed as the flow of energy through space or a medium in the form of linked, time-varying electric and magnetic fields. These fields oscillate at right angles to each other and perpendicular to the direction of motion. This model, derived from Maxwell’s equations, characterizes waves by their frequency and intensity, and it remains highly effective for describing phenomena like radio waves.12

Quantum Theory

Modern quantum theory, specifically quantum electrodynamics (QED), describes radiation as a flow of photons—discrete packets of energy defined by $E = h ν$ , where $h$ is Planck’s constant. In this framework, photons are excitations of quantized electric and magnetic fields and act as carriers of fundamental forces. While classical physics treats waves as continuous oscillations, quantum theory accounts for the wave-particle duality of light, where radiation exhibits both wavelike and particle-like properties depending on the interaction.1345