Shine light on a metal and knock electrons loose — the experiment that forced physics to accept the photon. This photoelectric effect simulator lets you change the light's wavelength, its intensity and the metal, then watch electrons fly across to the collector. The catch that broke classical physics: below a threshold frequency nothing comes off, however bright the beam. Dial in a retarding stopping voltage to measure the electrons' maximum energy, and read Planck's constant straight off the KEmax-versus-frequency line. It all runs on your device.
Drag the wavelength below through the threshold and watch the current switch on
Photon energy hν3.10 eV
Work function φ2.28 eV
Max KE of electrons0.82 eV
Stopping voltage0.82 V
Threshold wavelength544 nm
Photocurrent70%
Millikan's line: KEmax plotted against light frequency ν. It is flat zero below the threshold ν₀ = φ/h, then rises with slope equal to Planck's constant h. The glowing dot marks your current settings.
Frequency ejects electrons — brightness only counts them. One photon frees one electron, and a photon's energy is hν = 1240/λ (in eV, with λ in nm). If that falls short of the metal's work function φ, no electron escapes no matter how intense the light — turn the beam to full and the current stays dead. Cross the threshold and electrons appear at once, each with up to KEmax = hν − φ. Now intensity matters: more photons mean more electrons, a larger photocurrent — but never a faster one.
How It Works
Light as a stream of photons, a threshold that brightness cannot cross, and a stopping voltage that measures the rest.
1
One photon, one electron
The light is modelled as a stream of photons, each carrying energy hν = 1240/λ eV. When a photon reaches the metal it hands all of its energy to a single electron. Spending φ of that energy to break free of the surface, the electron leaves with whatever is left as kinetic energy — at most KEmax = hν − φ. Raising the intensity sends more photons, not more energetic ones.
2
The threshold
If a single photon carries less than the work function, it cannot free an electron at all, and piling on more such photons changes nothing. So there is a sharp threshold frequency ν₀ = φ/h — equivalently a threshold wavelength λ₀ = 1240/φ — below which the current is exactly zero. This is the anti-classical result: a wave should eventually shake an electron loose if you make it bright enough, but light does not work that way.
3
Stopping voltage and Planck's constant
Apply a reverse voltage at the collector and it drains the electrons' kinetic energy as they cross. The stopping voltage is the value that just halts the fastest electrons, so the current falls to zero — a direct read-out of KEmax = e·Vstop. Sweep the frequency and plot KEmax against it: the points fall on a straight line of slope h, exactly as Millikan measured to pin down Planck's constant.
What is the photoelectric effect?
The photoelectric effect is the emission of electrons from a metal surface when light shines on it. What makes it famous is that it does not behave like a wave should: below a certain frequency of light no electrons come off at all, no matter how bright the beam, and above that frequency electrons appear instantly even for very dim light. Einstein explained it in 1905 by treating light as particles — photons — each carrying an energy hν, and the result helped launch quantum physics.
Why does brighter light not always eject electrons?
Because each electron is knocked out by a single photon, and a photon's energy depends on the light's frequency, not its brightness. If one photon does not carry enough energy to free an electron — that is, if hν is less than the metal's work function — then adding more photons by turning up the intensity does not help; you just get more photons that are each individually too weak. Only raising the frequency (shortening the wavelength) increases the energy per photon. You can watch it here by keeping a low-frequency light at full intensity and seeing no current at all.
What is the work function and the threshold frequency?
The work function φ is the minimum energy needed to pull an electron out of a particular metal, measured in electron-volts. The threshold frequency ν₀ is the frequency at which a photon just barely carries that much energy, given by hν₀ = φ, and the matching threshold wavelength is λ₀ = 1240/φ nanometres. Light below the threshold frequency (longer wavelength) cannot eject electrons; light above it can. Different metals differ — caesium at about 2.1 eV releases electrons under visible light, while copper at 4.7 eV needs ultraviolet.
What is the stopping voltage?
The stopping voltage is the reverse voltage you must apply to the collector to just halt the most energetic electrons, so that the photocurrent drops to zero. Because the electrons' kinetic energy is spent climbing that voltage, the stopping voltage measures the maximum kinetic energy directly: KEmax = e·Vstop. Raising the light's frequency raises the stopping voltage; changing the intensity does not. Plotting KEmax against frequency gives a straight line whose slope is Planck's constant h — the measurement Millikan used to confirm Einstein's equation.