Photoelectric Effect Calculator
KEmax = hf − φ, and the threshold that broke classical physics.
Work out Photoelectric Effect. KEmax = hf − φ, and the threshold that broke classical physics. Says where the approximation stops holding.
Caesium 2.14, sodium 2.28, calcium 2.87, zinc 4.33, copper 4.70, platinum 5.65.
Maximum kinetic energy
0.8196 eV
Photon 3.1 eV · threshold 543.8 nm
That threshold behaviour is what broke classical physics. A wave theory predicts that brighter light should eventually shake an electron loose whatever its colour, and that is simply not what happens. Intensity controls how many electrons come out; frequency alone controls how fast they leave. The only way to explain it is that light arrives in discrete quanta of energy hf — which is what Einstein argued in 1905 and what his Nobel Prize was actually awarded for, not relativity. The stopping voltage in volts equals the maximum kinetic energy in electronvolts, which is exactly why the electronvolt is a convenient unit here.
How the Photoelectric Effect Calculator works
Enter a wavelength and a metal's work function for the maximum kinetic energy of the emitted electrons, the threshold wavelength and the stopping voltage. Below the threshold nothing is emitted, however bright the light — which is the whole point.
Also known as: work function calculator · threshold frequency calculator · stopping voltage calculator · maximum kinetic energy of photoelectrons
Frequently asked questions
What is the photoelectric equation?
KEmax = hf − φ. The photon's energy goes first into freeing the electron, costing the work function φ, and whatever is left becomes kinetic energy.
Why does brighter light not free electrons below the threshold?
Because intensity adds more photons, not more energy per photon. Each one is still individually too weak, and an electron cannot save up several. That is impossible on a wave theory and is exactly why quanta were needed.
What is the threshold frequency?
The frequency at which photon energy exactly equals the work function, so electrons are freed with zero energy to spare. For sodium at 2.28 eV, the threshold wavelength is 544 nm.
What is the stopping voltage?
The reverse voltage that just stops the fastest electron reaching the collector. In volts it equals the maximum kinetic energy in electronvolts, which is precisely why the electronvolt is the convenient unit here.
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