HSC Physics — Module 7
Photoelectric Effect — Flashcards & Quiz
The photoelectric effect is a pivotal experiment in HSC Physics Module 7 that provided evidence for the particle nature of light. When photons hit a metal surface, electrons are ejected only if the photon energy (E = hf) exceeds the work function (φ). Einstein’s explanation — that light consists of quantised packets of energy — contradicted the classical wave model. You need to calculate kinetic energy of emitted electrons using Eₖ = hf - φ and explain why increasing intensity increases current but not maximum kinetic energy.
Key Points
- Photons eject electrons from a metal surface only if photon energy E = hf ≥ work function φ (not intensity-dependent).
- Einstein's equation: hf = φ + ½mv_max² — the kinetic energy of ejected electrons depends on frequency, not intensity.
- Threshold frequency f₀ = φ/h; below this, no electrons are emitted no matter how bright the light.
- Stopping potential V_s measures max KE: eV_s = hf − φ. Plot V_s vs f to find h (gradient = h/e) and φ (x-intercept × h).
- Classical wave theory fails: predicts emission at any frequency given enough intensity; experiments show a threshold — this supports the photon model.
- Historical: Millikan confirmed Einstein's equation experimentally (1916), leading to Einstein's 1921 Nobel Prize (for the photoelectric effect, NOT relativity).
Common Mistakes to Avoid
- Claiming brighter light produces higher-energy electrons — no, brighter light just produces MORE electrons. Higher frequency gives higher-energy electrons.
- Forgetting the threshold frequency f₀ — below f₀, no electrons are emitted regardless of intensity or exposure time.
- Confusing kinetic energy of ejected electrons with the work function — KE_max = hf − φ, NOT hf − φ × intensity.
- Using wavelength in the Einstein equation directly — convert to frequency via c = fλ first.
- Mistaking the stopping potential V_s for the work function φ — they're related by eV_s = hf − φ, but not the same.
Exam Strategy
HSC Module 7 photoelectric questions often give you a graph of V_s vs f or KE_max vs f. The gradient gives h (Planck's constant), the x-intercept gives f₀ (threshold frequency), and the y-intercept gives -φ/e (work function). Know how to read each of these from a graph and how to calculate the numerical values. Historical context (Einstein's explanation beating the wave model) is often worth 2-3 marks.
Sample Flashcards
Q1: Describe the photoelectric effect and why it was significant.
When light above a threshold frequency shines on a metal surface, electrons are ejected instantly. Increasing intensity increases the NUMBER of electrons but NOT their maximum KE. Only increasing frequency increases KE_max. This could NOT be explained by classical wave theory.
Sample Quiz Questions
Q1: Increasing the intensity of light below the threshold frequency will eventually eject electrons.
Answer: FALSE
No matter how intense the light, if its frequency is below the threshold, no electrons are ejected. Each photon lacks sufficient energy (hf < φ) and photon energies do not add up.
Q2: In the photoelectric effect, electrons are emitted instantaneously when light above the threshold frequency strikes the surface.
Answer: TRUE
This contradicted wave theory, which predicted a time delay. Einstein's photon model explains it — a single photon transfers all its energy to one electron instantly.
Revision Tip
The Einstein equation and its graphical interpretation are exam gold — drill a Revizi deck of graph-reading questions until you can extract h, f₀ and φ in under a minute.
Related Concepts
Last updated: March 2026 · 1 flashcards · 2 quiz questions