WACE Physics — Unit 4
Photoelectric Effect — WACE Physics — Unit 4 Flashcards & Quiz
The photoelectric effect is the emission of electrons from a metal when light shines on it, and WACE Physics Year 12 Unit 4 treats it as the key experimental evidence for the photon model of light. Einstein's equation hf = φ + KE_max ties photon energy to the maximum kinetic energy of ejected electrons, and the threshold frequency represents the minimum frequency for any emission.
Key Points
- Photons are discrete packets of light energy: E = hf = hc/λ.
- Einstein equation: hf = φ + KE_max, where φ is the metal's work function.
- Threshold frequency f₀ = φ/h. Below f₀, no electrons are emitted no matter how bright the light.
- KE_max depends on photon FREQUENCY, not intensity. Higher intensity means more electrons, not higher energy per electron.
- Stopping potential: V_s = (hf – φ)/e. Plotting V_s vs f gives a straight line with gradient h/e.
- Classical wave theory fails to explain the threshold frequency and the instantaneous emission — the photon model is needed.
Common Mistakes to Avoid
- Claiming brighter light produces higher-energy electrons — it produces MORE electrons, not higher-energy ones.
- Forgetting the threshold frequency — no emission below f₀ regardless of exposure time or intensity.
- Using wavelength directly in hf — convert to frequency via c = fλ first.
- Confusing work function φ with stopping potential V_s.
- Failing to convert between eV and J when needed (1 eV = 1.6 × 10⁻¹⁹ J).
Exam Strategy
SCSA Unit 4 photoelectric questions ask you to calculate KE_max, work function, or interpret experimental data. Method: (1) identify photon frequency or wavelength, (2) apply hf = φ + KE_max, (3) for graphs, identify h from gradient and φ from intercepts. Historical context (Einstein winning the Nobel Prize for this) is often worth an extra mark.
Sample Flashcards
Q1: Describe the photoelectric effect and why wave theory fails.
Light ejects electrons from a metal. Wave theory predicts any frequency works with enough intensity. Observations: (1) threshold frequency f₀ required, (2) KE depends on frequency not intensity, (3) emission is instantaneous.
Q2: State Einstein's photoelectric equation.
E_k(max) = hf − W, where h = 6.63 × 10⁻³⁴ J s, f is frequency and W (also written φ) is the work function — the minimum energy to remove an electron from the surface. At threshold: E_k = 0, so W = hf₀.
Q3: How is Planck's constant found from a stopping voltage experiment?
A reverse voltage is raised until the current stops; then eV_stop = E_k(max) = hf − W. Repeat for several frequencies and plot V_stop (or E_k) against f: a straight line whose gradient is h/e (or h), whose x-intercept is f₀ and whose y-intercept is −W/e (or −W).
Sample Quiz Questions
Q1: Increasing light intensity increases photoelectron kinetic energy.
Answer: FALSE
Intensity increases the NUMBER of electrons, not their energy. KE depends on frequency.
Q2: Below the threshold frequency, no electrons are emitted regardless of intensity.
Answer: TRUE
Each photon must have energy ≥ W. Below f₀, individual photon energy is insufficient.
Q3: The photoelectric effect provides evidence for the particle nature of light.
Answer: TRUE
Requires photons to explain threshold frequency and instantaneous emission.
Revision Tip
Einstein equation problems are formulaic — drill a Revizi deck with 10+ photoelectric calculations varying which quantity is the unknown.
Related Concepts
Last updated: 9 October 2026 · 3 sample flashcards · 3 sample quiz questions