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.
You are in the Physics lab.
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.
What the photon beam, the retarding voltage and the Millikan graph are telling you.
The lamp fires photons toward the cathode, and their colour is set by the wavelength slider — true spectral tints across the visible range, deep violet for ultraviolet. Each photon carries energy hν = 1240/λ eV. It only frees an electron if that energy clears the metal's work function, the emission gate hν ≥ φ; fall below it and the cathode dims, a red banner reads "no electrons ejected", and turning intensity to full just sends more photons that are each too weak. Cross the gate and glowing electrons stream to the anode.
The battery in the loop applies a reverse voltage that drains each electron's kinetic energy as it crosses the gap. Raise the retarding voltage and the slowest electrons fall short and reverse before reaching the anode, so the ammeter needle drops. The exact voltage that halts even the fastest electrons — where the current just reaches zero — is the stopping voltage, and it reads out the maximum kinetic energy directly, KEmax = e·Vstop.
The lower plot is Millikan's line: KEmax against light frequency ν. It stays flat at zero until the threshold ν₀ = φ/h — the dashed marker — then climbs as a straight line whose slope is Planck's constant h, intercepting the axis exactly at ν₀. The glowing dot is your current settings; slide the wavelength and watch it travel along the line, or switch metals and see the whole line shift its threshold.
Light as a stream of photons, a threshold that brightness cannot cross, and a stopping voltage that measures the rest.
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.
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.
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.
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A ready-to-assign lab using the simulation above. Free to use — nothing to install or sign up for, and no student data leaves the browser.
Record whether electrons are emitted versus frequency and intensity, and their maximum kinetic energy.
Light delivers energy in photons of energy hf. Electrons escape only when a single photon carries at least the metal’s work function, so there is a threshold frequency independent of intensity — direct evidence for the particle nature of light (KEmax = hf − φ).
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