Quantum physics in SPM Physics asks you to explain light as photons and to use the photoelectric equation. This cluster is part of SPM Physics and stresses interpretation as much as calculation.
What does the cluster cover?
Begin with explaining the photoelectric effect, then relate photon energy, frequency and wavelength.
Next, read threshold-frequency graphs. The final lesson, distinguishing photon explanations from classical expectations, is about writing the explanation.
What does a graph-to-equation example look like?
An original graph plots the maximum kinetic energy of emitted electrons against light frequency. It is a straight line that meets the frequency axis at 5.0 × 10¹⁴ Hz and passes through 8.0 × 10¹⁴ Hz at 1.99 × 10⁻¹⁹ J.
The equation is KE(max) = hf − W, which is a straight line in f. So the gradient of the graph is h.
Gradient = 1.99 × 10⁻¹⁹ ÷ (8.0 × 10¹⁴ − 5.0 × 10¹⁴) = 1.99 × 10⁻¹⁹ ÷ 3.0 × 10¹⁴ = 6.6 × 10⁻³⁴ J s.
For the work function, use the point where KE(max) is zero. There hf = W, so W = h × 5.0 × 10¹⁴ = 6.63 × 10⁻³⁴ × 5.0 × 10¹⁴ = 3.3 × 10⁻¹⁹ J.
The choice worth explaining is why the intercept on the frequency axis gives the threshold frequency. It is the only point where emission just begins, so the kinetic energy is zero. The vertical intercept would be −W, which a plotted graph does not show.
| Observation | Classical wave idea | Photon model |
|---|---|---|
| Below threshold frequency | Bright enough light should still free electrons | No single photon has enough energy |
| Brighter light above threshold | Electrons should gain more energy each | More photons, so more electrons, same maximum energy |
Where should you start?
If your powers of ten keep slipping, use the units and significant figure checker first. If graphs are the weak point, go straight to the threshold lesson.
Then try the cluster practice set. To have a teacher check your graph readings and units with you, see online one-to-one Physics tuition.