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E.2.1 (HL)—Photoelectric effect

Syllabus
First assessment 2025
Objective
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Level
HL

Interpret the Photoelectric Effect

HL only

Read the observations

When monochromatic light illuminates a metal, electrons may be emitted. Increasing intensity increases the emission rate, but for fixed frequency it does not increase the maximum kinetic energy of the emitted electrons.

Use the photon model

Light transfers energy in individual photons. One photon interacts with one electron, so photon frequency sets the energy available per interaction, while intensity changes the number of photons arriving per second.

Identify the evidence

The intensity–energy distinction and the existence of a threshold frequency cannot be explained by a simple continuous wave-energy model. They support the particle nature of light.

Common trap

Do not say that brighter light makes each photoelectron more energetic. At fixed frequency, it produces more emitted electrons, not a larger maximum kinetic energy.

E.2.1 (HL) Exam Analysis

HL only

Assessment in practice

1–2 marks
How it is assessed

Questions compare changes in emission rate and kinetic energy after changing intensity, or identify which observations conflict with the wave model.

Command terms

Identify / Compare

What earns marks

Separate photon number from photon energy and state that the threshold condition is set by frequency, not intensity.

Watch for

Claiming that increased intensity raises maximum kinetic energy at fixed frequency.

Retrieve the Quantum Model

HL only

Retrieve the light model

The photoelectric effect and Compton scattering show photon-like energy and momentum transfer. Threshold frequency and Ek,max=hf−ΦE_{k,max}=hf-\Phi make the photon energy budget explicit.

Retrieve the matter model

Particle diffraction and λ=h/p\lambda=h/p show wave-like matter. For Compton scattering, track energy loss, increased wavelength, and Δλ=hmec(1−cos⁡θ)\Delta\lambda=\frac{h}{m_ec}(1-\cos\theta).