Einstein and the Photoelectric Effect

What happened

In 1905 the young Albert Einstein proposed that light itself comes in quanta — particle-like packets later called photons. This explained the puzzling photoelectric effect, in which light knocks electrons out of metal only if its colour (frequency) is high enough, no matter how bright it is.

Why it matters

Einstein's light quanta showed that the quantum idea was real physics, not a mathematical trick, and helped found quantum mechanics. It was for this work — not relativity — that he was awarded the 1921 Nobel Prize.

Sources

• The Nobel Prize. Albert Einstein – Facts (Nobel Prize in Physics 1921) · Reputable source