Wave theory treats light as a continuous stream of energy. The photon model treats it as packets of energy hf, and only the photon model matches what the photoelectric experiment shows.
This lesson is part of SPM Physics. It brings together explaining the photoelectric effect and reading threshold-frequency graphs.
What does each model predict?
| Question | Wave theory predicts | Experiment shows | Photon explanation |
|---|---|---|---|
| Brighter light, same colour | Faster electrons | Same maximum KE, more electrons | More photons, each with the same energy |
| Low frequency, very bright | Electrons emitted eventually | No emission below f₀ | Each photon is below W |
| Dim light, high frequency | Delay while energy builds up | Immediate emission | One photon frees one electron at once |
| Higher frequency | No change to KE | Higher maximum KE | Photon energy hf is larger |
Worked example: which beam ejects electrons?
An original metal has work function 3.2 × 10⁻¹⁹ J. Beam P is dim blue light of frequency 7.0 × 10¹⁴ Hz. Beam Q is bright red light of frequency 4.0 × 10¹⁴ Hz.
Photon energy of P = 6.63 × 10⁻³⁴ × 7.0 × 10¹⁴ = 4.6 × 10⁻¹⁹ J, which exceeds the work function. Electrons are emitted at once, even though the beam is dim.
Photon energy of Q = 2.65 × 10⁻¹⁹ J, which is below the work function. No electrons are emitted, and brightness does not change that.
How do I write the explanation?
Use a four-step frame, in this order.
- Light consists of photons, each with energy E = hf.
- One photon gives all its energy to one electron.
- Emission needs hf ≥ W, so the frequency must reach the threshold.
- State the effect on the number of electrons or their energy.
The mistake that costs marks
The slip is to claim that increasing the intensity increases the kinetic energy of each electron.
| Change | Wrong answer | Right answer |
|---|---|---|
| Double the intensity | KEmax doubles | KEmax unchanged, current doubles |
| Double the frequency | Number of electrons doubles | KEmax increases, since hf is larger |
Intensity changes how many photons arrive each second. Frequency changes how much energy each photon carries.
Check yourself
Light of frequency above the threshold falls on a metal and a current flows. Explain what happens to the current and the maximum kinetic energy when the light is made three times brighter with the same frequency.
Answer
Three times brighter means three times as many photons arrive each second, so three times as many electrons are emitted each second. The current increases to three times its value.
Each photon still has energy hf, so each electron still receives the same energy. The maximum kinetic energy is unchanged.
What to study next
Test all four skills in the quantum physics practice set. If photon-energy calculations are the weaker part, return to relating photon energy, frequency and wavelength.
To practise writing photon explanations with a teacher, see online one-to-one Physics tuition.