The same light can cause photoelectric emission from one metal and leave another without emission. In the single-photon model, a photon must supply at least the work function of the surface for an electron to escape.
Compare photon energy hf with each work function before calculating kinetic energy. When emission occurs, the maximum kinetic energy is hf − Φ.
- Calculate the energy of one photon from its frequency.
- Compare that energy with each metal’s work function.
- Use consistent energy units when subtracting the work function.
Worked example | Two work functions under one light source
Monochromatic light of frequency 6.00 × 10¹⁴ Hz shines on metal A with work function 2.10 eV and metal B with work function 3.20 eV. Using h ≈ 4.14 × 10⁻¹⁵ eV s gives a photon energy of about 2.48 eV.
The photon energy exceeds 2.10 eV, so metal A emits photoelectrons with maximum kinetic energy about 0.38 eV. It falls below 3.20 eV, so metal B gives no photoelectric emission under this model.
What changes if the light is brighter?
At the same frequency, increasing intensity increases the number of photons arriving per second while the energy of each photon stays the same. For metal A, this can increase the rate of photoelectron emission. The maximum kinetic energy stays the same.
For metal B, the individual photon energy is still below the work function. To reach the threshold in the single-photon model, increase the frequency.
Raise the frequency
For the same two metals, use light of frequency 8.00 × 10¹⁴ Hz. Determine which metals emit and estimate the maximum kinetic energy for each.
Answer. The photon energy is about 3.31 eV. Both metals emit. The maximum kinetic energies are approximately 1.21 eV for A and 0.11 eV for B, using the supplied values.
Frequently asked questions
What happens exactly at the threshold frequency?
In the ideal model, the photon energy equals the work function and the maximum kinetic energy is zero.
Which metal has the higher threshold frequency?
Metal B. Since f₀ = Φ ÷ h, its larger work function requires a higher threshold frequency.
Official information
SEAB, 2027 GCE A-Level H2 Physics syllabus (9478), Quantum Physics, photoelectric effect. Syllabus checked 11 November 2026. The examples are original.