Science › Quantum world
Photoelectric effect: light comes in particles
1905· Discovery / experiment
ProvenTested many times; part of working physics.
Light shining on metal knocks electrons loose, but only if each bit of light carries enough energy; brightness alone does not help.
See it on the Science map →How it works
If light were only a wave, bright enough light of any color should eventually free electrons. Instead, below a certain frequency nothing happens, however bright. Above it, electrons fly out instantly, with energy set by the color, not the brightness. In 1905 Einstein explained why: light comes in particles, later called photons. One photon hits one electron. Brightness only changes how many photons arrive.
Why it matters
It made light quanta physical, not just a math trick. Einstein's 1921 Nobel Prize was for this, and it underpins solar panels, digital cameras and light sensors.
Think of it like
Like a door that needs one strong shove: a thousand light taps never open it, but a single hard push does.
How we know
Robert Millikan set out to disprove Einstein and instead confirmed his prediction precisely in 1914–1916. Arthur Compton's 1923 X-ray scattering showed photons also carry momentum, like particles.
Connections (3)
▸Albert Einstein explained (his Nobel Prize) Photoelectric effect: light comes in particlesProven
The relationship
Einstein took Planck's packets seriously as physical things, proposing that light itself travels as quanta, each with energy set by its frequency. One quantum frees one electron, so color, not brightness, decides whether electrons escape and how fast. This explained 1902 measurements by Philipp Lenard that wave theory could not.
How the link was found
Einstein, in a March 1905 paper, part of his 'miracle year' that also produced special relativity. He was a 26-year-old patent clerk in Bern.
Why it's proven
Millikan's careful 1914–1916 experiments confirmed Einstein's prediction exactly. Compton scattering (1923) showed photons carry momentum. Einstein's 1921 Nobel Prize cites this work, not relativity.
Open questions and mysteries
Einstein himself described the light-quantum idea as very revolutionary, yet later stayed uneasy with the quantum theory it helped create.
▸Energy comes in packets (quanta) was applied to light in Photoelectric effect: light comes in particlesProven
The relationship
Planck had applied packets only to how hot matter emits energy. Einstein extended the idea to light itself: a light beam is a stream of energy packets. Because each packet's energy depends on frequency, only light above a threshold color can free an electron, exactly the puzzling photoelectric behavior.
How the link was found
Einstein, 1905, in his paper on light quanta. The word 'photon' came later, coined in 1926 by chemist Gilbert Lewis.
Why it's proven
Millikan's 1914–1916 measurements extracted Planck's constant from photoelectric data, matching the value from oven radiation: the same constant from two very different experiments.
Open questions and mysteries
Light's particle nature coexists with its wave nature, a tension resolved only mathematically, in quantum field theory.
▸Photoelectric effect: light comes in particles showed light is also particles, part of Wave–particle dualityProven
The relationship
Light was firmly established as a wave by the 1800s. The photoelectric effect showed it also arrives in particle-like packets, so light was both, depending on the experiment. In 1924 de Broglie reasoned that if waves can act like particles, particles like electrons should act like waves, completing the duality.
How the link was found
Einstein raised light's dual nature in 1905 and argued in 1909 that light must have both wave and particle features. Louis de Broglie extended it to matter in his 1924 PhD thesis.
Why it's proven
Photoelectric and Compton experiments show particle behavior; interference shows wave behavior of the same light. Electron diffraction in 1927 confirmed de Broglie's matter waves.
Open questions and mysteries
How one object can be both is answered by math, not intuition; what is 'really there' between detections depends on interpretation.