Skip to content

Salam Academy

Sign in Create account

LearnCambridge97022025–2027Paper 4 · A Level Structured QuestionsQuantum physics

Paper 4 · Module

Quantum physics

3 subtopics≈ 110 min in total

Start the module: Photons and the photoelectric effect Open graphing calculator

Photons and the photoelectric effect
Light comes in quanta called photons, each of energy $E=hf=\dfrac{hc}\lambda$ ( $h=6.63\times10^{-34}$ J s) and momentum $p=\dfrac Ec$ . When photons hit a metal, each can release one electron if its energy exceeds the work function $\Phi$ : $hf=\Phi+\tfrac12mv_{\max}^2$ . Below the threshold frequency $f_0=\dfrac\Phi h$ , no electrons are emitted, however bright the light. Brighter light means more photons, so more electrons, but not faster ones.
≈ 40 min
Wave–particle duality
Light behaves as a wave (diffraction, interference) and as particles (the photoelectric effect). Matter does too: de Broglie proposed that any moving particle has a wavelength $\lambda=\dfrac hp=\dfrac h{mv}$ . Electron diffraction through thin graphite proves it: electrons form rings like waves passing through a grating. Faster electrons have shorter wavelengths, so the rings get smaller.
≈ 35 min
Energy levels and line spectra
Electrons in an isolated atom can only have certain discrete energies, called energy levels (negative values, with zero meaning the electron is free). When an electron drops from level $E_1$ to a lower level $E_2$ , it emits a photon with $hf=E_1-E_2$ . Because only certain differences exist, only certain wavelengths appear: a line spectrum. An absorption spectrum shows dark lines where the atoms absorb exactly those photon energies from a continuous spectrum.
≈ 35 min