Photons — Flashcards & Quiz

Key Points

• Photon energy: E = hf = hc/λ, where h = 6.63 × 10⁻³⁴ J s is Planck's constant.
• Photoelectric effect: when light shines on a metal, electrons are ejected only if photon energy ≥ the work function φ.
• Einstein equation: hf = φ + KE_max. Kinetic energy depends on frequency, not intensity.
• Threshold frequency f₀ = φ/h — below this, no electrons are ejected regardless of intensity.
• Stopping potential V_s relates to max KE via eV_s = hf – φ. Plotting V_s vs f gives a line with gradient h/e.
• Wave theory cannot explain threshold frequency or the instantaneous emission — the photon model is needed.

Common Mistakes to Avoid

1. Claiming brighter light produces higher-energy electrons — brighter means MORE electrons, not higher energy per electron.
2. Forgetting the threshold frequency — below f₀, no electrons are ejected even with very bright light.
3. Using wavelength directly in hf — convert to frequency first via c = fλ.
4. Mixing up stopping potential V_s with work function φ — they are related but distinct.
5. Assuming intensity matters for individual electron energy — it only matters for the number of electrons.

Exam Strategy

SACE Stage 2 photon questions usually give you experimental data (threshold frequency, stopping potential) and ask you to calculate work function, Planck's constant, or maximum KE. Method: (1) identify knowns, (2) apply hf = φ + KE_max, (3) convert between wavelength and frequency using c = fλ as needed. Be ready to plot and interpret V_s vs f graphs.

Revision Tip

Einstein equation problems are formulaic — drill a Revizi deck with 10+ photoelectric calculations varying the given and unknown quantities.