Einstein's photoelectric law, basis of modern light tech

Person: Albert Einstein
Lived: 1879–1955
Field: Physics
From: German-American

The story

Albert Einstein was born in 1879 into a Jewish family in Ulm, Germany, and grew up in Munich, where his father ran an electrical engineering company. After studying in Zurich he took a job at the Swiss Patent Office in Bern, and it was there, as a 26-year-old patent examiner, that he published four groundbreaking papers in 1905, sometimes called his miracle year.

The first tackled a puzzle about light and metal. Since Heinrich Hertz's observations in 1887, physicists had known that light shining on a metal surface can knock out electrons. In 1902 Philipp Lenard found that the energy of those electrons did not depend on how bright the light was, which made no sense if light was simply a wave. Below a certain frequency, even intense light released no electrons at all.

Einstein built on Max Planck's idea that energy is emitted in packets. He proposed that light itself travels as discrete quanta, now called photons, each carrying energy proportional to its frequency. A single quantum hands its energy to a single electron, so only light above a threshold frequency can set electrons free. It was a radical idea that contradicted the wave theory of light then accepted.

The theory predicted that the energy of the ejected electrons should rise in a straight line with the frequency of the light. In 1914 the American physicist Robert Millikan confirmed the prediction, and in 1923 Arthur Compton's X-ray experiments persuaded more physicists that light quanta were real. The idea that light behaves both as a wave and as particles became a cornerstone of quantum mechanics.

Einstein was awarded the 1921 Nobel Prize in Physics, presented in 1922, "for his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect". The photoelectric effect was the only specific discovery named in the citation. At the time he was affiliated with the Kaiser Wilhelm Institute for Physics in Berlin.

The photoelectric effect is now at work in photomultiplier tubes that detect faint flashes of light, in night-vision devices and in many kinds of light sensors. In 1917 Einstein built further on the idea of light quanta when he described stimulated emission, the process behind the laser.

Why it matters

Einstein's idea of light quanta helped found quantum physics and underlies technologies from sensitive light detectors and night-vision equipment to lasers.