The Photoelectric Effect and the Work Function
Electrons leave a metal surface only above a threshold frequency, however bright the light gets, which no wave picture accounts for. One photon frees one electron, giving hf equal to the work function plus the greatest kinetic energy.
What a learner can do afterwards
- Names the observations a wave model of light cannot explain
- Uses the photoelectric equation to find a threshold frequency or a maximum kinetic energy
- Explains why brighter light frees more electrons but not faster ones
1 · Read
Shine light on some metals and electrons pop out at once. A wave picture fails three tests. Dim blue light frees electrons instantly, while bright red light frees none at all, however long you wait. The top speed of the freed electrons depends on colour, not brightness.
Think of each photon as one coin. One photon meets one electron and pays an exit fee called the work function. What is left becomes the electron's top kinetic energy. A 6.0e-19 J photon hitting metal with a 4.0e-19 J work function frees an electron with 2.0e-19 J to spare. The split works both ways: photon energy equals exit fee plus leftover kinetic energy.
Turn up the brightness without changing the colour and more photons arrive each second. More electrons leave each second, but each one still meets a photon with the same energy, so the top speed stays fixed. Change to a higher frequency instead and each photon carries more energy, so freed electrons can leave faster.
When no electrons leave, check the frequency before the brightness. Below the threshold frequency the photons cannot pay the exit fee, so nothing escapes however bright the beam. A dimmer but bluer lamp is the fix, not a brighter red one.
One photon frees one electron when its frequency clears the threshold, and brightness only sets how many leave.
2 · Watch
Take it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.