Light strikes a metal one photon at a time. An electron escapes only if a single photon carries at least the work function Φ. Below the threshold frequency, no electrons are emitted no matter how bright the light. Above it, the maximum kinetic energy is hf − Φ, and brighter light ejects more electrons per second.
MissionFind the threshold: lower the wavelength until electrons just begin to escape, then raise the intensity and watch only the rate change.Streak 0Best 0
Keeping the frequency fixed, increasing the intensity increases:
metal · work function ΦCaesium · 2.1 eV
photon energy hf2.76 eV
threshold λ₀ = hc/Φ590 nm
max KE = hf − Φ0.66 eV
stopping voltage0.66 V
Emission occurs because each photon carries more than the work function.