Write Einstein's photoelectric equation. Explain how it accounts for the laws of photoelectric emission. Draw the graph of stopping potential against frequency.

1. 1 · State the photon idea. Light is made of photons, each of energy hν; one photon is absorbed by one electron.
2. 2 · Write the equation. hν = φ₀ + K_max, so K_max = h(ν − ν₀) and eV₀ = hν − φ₀.
3. 3 · Explain the threshold. If hν < φ₀, no electron can escape, so there is a threshold frequency ν₀ = φ₀/h.
4. 4 · Explain intensity and frequency. More intensity means more photons: more current but the same K_max. Higher ν means a larger K_max and V₀.
5. 5 · Explain no time lag. Each photon hands over its energy at once, so emission is instantaneous.
6. 6 · Draw the V₀ against ν graph. [Graph: straight line starting at ν₀ on the frequency axis with slope h/e; dashed extension meeting the V₀ axis at −φ₀/e]
7. 7 · Real life. Mobile phone camera sensors and solar cells work on this one-photon, one-electron rule.
8. 8 · Close in one line. So, the photon picture explains every law that the wave theory could not.

Scoring tip: Write the equation first, then give one line for each law. Label ν₀ and the slope h/e on the graph.