Photoelectric Effect (Einstein)
In 1905 Einstein explained why light knocks electrons out of a metal only above a certain frequency: light arrives in packets of energy, later called photons, and one packet frees one electron.
Open in the interactive tree →Hertz saw in 1887 that ultraviolet light helps sparks jump, and in 1902 Philipp Lenard found that the energy of the freed electrons does not depend on how bright the light is. Einstein proposed that light carries energy in packets E = hf, so an electron is freed only if one packet carries enough energy; Millikan spent about a decade testing the equation (first report 1914) and confirmed it in every detail, and Einstein received the Nobel Prize in Physics for 1921 for this law. The photon picture underlies solar cells and light sensors, and Einstein's 1917 theory of stimulated emission, the basis of the laser, describes light the same way.
Prerequisites
- Radio Waves Proven (Hertz)1886-1888Hertz first saw ultraviolet light help sparks jump, in 1887
- Discovery of the Electron1897The particles knocked out of the metal are electrons
- Planck's Law (Energy Quanta)1900Einstein applied Planck's energy packets to light itself
Unlocks
- Quantum Mechanics1925Light quanta were the first extension of Planck's energy packets beyond heat radiation
- Solar Cells (Photovoltaics)1954Einstein's explanation of light freeing electrons underlies the solar cell
- Laser1960Einstein's 1917 stimulated emission rests on his photon picture of light
- CCD Image Sensor1969A CCD pixel turns arriving photons into electric charge