Photoelectric effect and light quanta

What it shows

The photoelectric effect shows that light delivers energy in packets called photons. Each photon carries energy E = hf = hc/λ, and it can free an electron only if that energy is at least the work function φ of the metal. The maximum kinetic energy of the ejected electrons is Ek(max) = hf − φ, so it depends on the frequency of the light, not on its brightness. Below the threshold frequency f₀ = φ/h, that is above the threshold wavelength λ₀ = hc/φ, no electrons escape however intense the light is, which the classical wave model cannot explain.

How to use

Pick a Cathode metal, then move Light wavelength slowly from red towards ultraviolet and note where electrons first appear; compare this with λ₀ in the readout. Next, keep the wavelength fixed and change Light intensity: watch whether the number of electrons or their energy changes. Finally, repeat with another metal and compare the Ek(max) against frequency lines: same slope, different intercept on the frequency axis.

Parameters you can change

• Cathode metal Caesium (φ = 2.14 eV), Sodium (φ = 2.28 eV), Zinc (φ = 4.30 eV), Copper (φ = 4.70 eV)
• Light wavelength 150–700 nm
• Light intensity 10–100 %

Questions to explore

1. Why does increasing the intensity of red light not eject any electrons from sodium?
2. When you change the metal, which feature of the Ek(max)–frequency graph changes and which stays the same?
3. Given its work function of 4.30 eV, what is the threshold wavelength of zinc, and does the simulation agree?