Mustafa Bakr

A metal sphere is a trap for light waves. Bouncing off the walls, they combine into frozen oscillations — like crests on a pond, frozen in time. For each such pattern (mode), Maxwell's equations prescribe a strict frequency.

Quantum electrodynamics, or QED, describes how light interacts with charged particles — for example, electrons. It is one of the most precise theories in history: its predictions match experiment to the eleventh decimal place.