Classical physics predicts that brighter light ejects faster electrons. Experiment says otherwise: below a threshold frequency, no electrons escape regardless of intensity; above it, energy depends on frequency not brightness. Einstein's 1905 explanation — light is quantised — won him the Nobel Prize.
Photons carry energy E = hf (Planck constant × frequency). An electron escapes the metal only if E > ϕ (the work function). Kinetic energy of ejected electrons = hf − ϕ, independent of intensity. Intensity controls the number of photons per second, hence the photocurrent.
Adjust Frequency to cross the threshold and watch electrons begin to escape. Raise Intensity to increase photocurrent without changing electron energy. Switch metal to change ϕ. Observe the live K.E. vs Frequency graph — its slope is Planck's constant h.
When Philipp Lenard measured the photoelectric effect in 1902, he expected wave theory to hold — but it didn't. Einstein's particle explanation was so radical that even after Millikan's precise 1916 confirmation, Millikan himself refused to accept photons were real for another decade.
This interactive model shines a beam of light at a metal plate and shows the photoelectric effect in real time. Each incoming photon carries energy E = hf, where h is the Planck constant and f the light's frequency. An electron is ejected only when this energy exceeds the metal's work function W; the freed electron then leaves with kinetic energy hf − W. A live energy diagram plots the photon bar, the work-function threshold and the resulting kinetic-energy bar in electronvolts.
The simulation applies Einstein's photoelectric equation. Photon energy is computed as E = h·f with h = 4.136×10⁻¹⁵ eV·s, the work function W is fixed per material (2.1 eV for caesium up to 5.1 eV for gold), and kinetic energy is max(0, hf − W). Below threshold, photons strike the plate but produce only a red flash; above it, glowing electrons spray from the surface.
Drag the Frequency slider (4–10 ×10¹⁴ Hz) to change photon energy and beam colour. Use the Intensity slider (1–10) to add more photons per second, raising the electron rate without altering their energy. Pick a Material to set the work function, and press Pause to freeze the scene. The bottom bar reports photon energy, W, kinetic energy and electrons per second.
Einstein received the 1921 Nobel Prize in Physics specifically for explaining the photoelectric effect, not for relativity. His 1905 proposal that light is quantised into discrete packets was so radical that Robert Millikan spent a decade trying to disprove it before his own precise measurements confirmed it instead.
It is the emission of electrons from a material when light shines on it. The simulation shows photons hitting a metal plate; if each photon carries enough energy, an electron is knocked free and flies off. It was the key evidence that light behaves as discrete quanta rather than only as a continuous wave.
Each photon delivers energy E = hf in a single interaction with one electron, so a higher frequency means a more energetic photon. Increasing intensity adds more photons but does not make any single one more energetic. That is why turning up the Intensity slider raises the electron count but never lifts an electron over the threshold if the frequency is too low.
The Material selector sets the work function W, the minimum energy needed to free an electron from that metal. Caesium has the lowest value at 2.1 eV and gold the highest at 5.1 eV. Emission occurs only when the photon energy hf exceeds W, which is why high work-function metals like gold require much higher frequencies before any electrons appear.
The energy bookkeeping follows Einstein's equation exactly: photon energy hf, threshold W, and kinetic energy hf − W are all correct in electronvolts using h = 4.136×10⁻¹⁵ eV·s. The work-function values are realistic literature figures. The flying particles are a stylised visualisation, so electron speeds and trajectories are illustrative rather than a precise scattering model.
An electron is bound inside the metal and needs a minimum amount of energy, the work function, to escape. Because a photon transfers its whole energy hf to one electron at once, no emission happens until hf reaches W. Below that point the energy is simply absorbed and re-emitted, which the simulation marks with a red flash instead of an ejected electron.