Spotted in 1887 by the physicist who was busy proving radio waves exist (1887)
Heinrich Hertz noticed sparks jumped more easily under ultraviolet light
Science · Physics
The photoelectric effect
1887; explained 1905
Physics — quantum theory
The emission of electrons from a metal surface when light shines on it. Heinrich Hertz noticed it in 1887, and Philipp Lenard showed that the energy of the ejected electrons depends on the light's frequency, not its brightness — something classical wave theory could not explain. In one of his 1905 "miracle year" papers Albert Einstein solved it by proposing that light itself comes in quanta, each carrying energy hf; below a threshold frequency no electrons come out at all, however intense the beam. Robert Millikan spent a decade trying to disprove the equation and ended up confirming it, and it was for this, not relativity, that Einstein received the Nobel Prize.
How to spot them
- ▸Noticed by Hertz in 1887 while he was generating radio waves
- ▸Explained in a 1905 paper by a Bern patent clerk, and the work for which he won his Nobel Prize
- ▸Brighter light gives more electrons but not faster ones; below a threshold frequency, none at all
- ▸Behind solar cells, photomultipliers, night-vision tubes and the light meter in a camera
Key facts
The subject of the 1905 paper for which Einstein was actually awarded the Nobel Prize (1905)
Announced in 1922 as the prize for 1921; relativity was pointedly not mentioned
The phenomenon that forced physicists to accept the light quantum, later called the photon
Gilbert Lewis coined 'photon' in 1926
Millikan spent ten years trying to refute the equation and confirmed it instead (1916)
His precise measurements also gave a good value for Planck's constant
The minimum energy needed to free an electron from a metal, called the work function
Caesium's is low, around 2 eV, which is why it is used in photocells
Philipp Lenard's 1902 discovery that electron energy depends on colour, not brightness (1902)
Lenard won the 1905 Nobel Prize for his work on cathode rays
Key ideas
- Light delivers its energy in discrete packets of size hf, so one packet frees one electron; the leftover energy after paying the work function becomes the electron's kinetic energy.
- Turning up the brightness sends more packets, not bigger ones, so it releases more electrons but never faster ones — the fact classical wave theory could not accommodate.
Quick-fire facts
- •Heinrich Hertz observed it in 1887; his assistant Wilhelm Hallwachs studied it further, so it is sometimes called the Hallwachs effect
- •Philipp Lenard established in 1902 that the electrons' maximum energy depends on frequency, not intensity
- •Einstein's explanation came in his March 1905 paper, the first of the annus mirabilis papers
- •Einstein's equation: maximum kinetic energy = hf − φ, where φ is the work function and h is Planck's constant
- •The 1921 Nobel Prize in Physics, presented in 1922, cited 'his discovery of the law of the photoelectric effect' — not relativity
- •Robert Millikan verified the equation experimentally by 1916 and won the 1923 Nobel Prize partly for it, despite having doubted the light-quantum idea
- •The threshold frequency, and so the work function, varies with the metal: caesium and potassium emit under visible light, most metals need ultraviolet
- •Emission is essentially instantaneous, another thing wave theory got wrong, which predicted a delay while energy accumulated
- •Applications include photomultiplier tubes, image intensifiers, photoelectron spectroscopy (XPS) and the photoelectric smoke detector's cousin, the photocell
- •A common misconception is that solar panels work by this effect; most work by the photovoltaic effect inside a semiconductor junction, a related but distinct process
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Sources
- The Nobel Foundation — The Nobel Prize in Physics 1921 Awarded to Einstein for the law of the photoelectric effect
- Abraham Pais — Subtle Is the Lord: The Science and the Life of Albert Einstein On the 1905 light-quantum paper and its reception