IB Physics: The Photoelectric Effect — Common Questions Answered

Most lost marks come from explaining the effect using wave theory instead of photon theory — saying light's "intensity" causes emission rather than a single photon's energy exceeding the work function. Examiners want the particle-nature argument stated explicitly, with correct use of threshold frequency, not just the formula copied down.

It's the emission of electrons from a metal surface when light of sufficiently high frequency hits it. Below a certain threshold frequency, no electrons come out at all, no matter how bright the light — proof that light behaves as discrete photons, each carrying energy $E=hf$, not as a continuous wave.

Wave theory predicts that any frequency of light, given enough time or intensity, should eventually eject electrons — it doesn't. Instead, there's a sharp threshold frequency below which nothing happens, and emission above it is instant. Only treating light as photons, each with fixed energy $hf$, explains both observations.

The work function ($\varphi$) is the minimum energy needed to remove an electron from a specific metal's surface, measured in joules or electron volts. Threshold frequency ($f_0$) is the minimum light frequency that supplies exactly that energy, related by $\varphi =h{f}_0$. Different metals have different values for both.

The equation is $hf=\varphi +E_{k(max)}$, where $hf$ is the incoming photon's energy, $\varphi$ is the work function, and $E_{k(max)}$ is the maximum kinetic energy of an emitted electron. Rearrange it to find whichever variable the question asks for — it's a straight rearrangement, not a derivation.

On a graph of maximum kinetic energy ($E_k$) against frequency ($f$), the gradient equals Planck's constant $h$, the y-intercept is $-\varphi$, and the x-intercept is the threshold frequency $f_0$. Examiners frequently ask you to find $h$ from a given graph, so know exactly what each feature represents.

Stopping voltage ($V_s$) is the potential difference needed to stop even the fastest emitted electrons reaching the collector, linked to kinetic energy by $e{V}_s=E_{k(max)}$. Threshold frequency is the minimum frequency of light needed to cause emission at all. One measures electron energy; the other measures whether emission happens.

Because intensity only increases the number of photons hitting the surface per second, not the energy of each individual photon. Since each electron is ejected by absorbing exactly one photon, more photons mean more electrons emitted (higher current), but the maximum kinetic energy per electron stays fixed by $hf-\varphi$.

State the observation (threshold frequency exists, emission is instantaneous), explain why wave theory fails to predict it, then explain how photon theory does — using $E=hf$ explicitly. Command terms like "explain" require reasoning linked to a physical model, not just a formula quoted without context.

Confusing work function with threshold frequency, forgetting to convert electron volts to joules before using $h$ in SI units, and describing intensity as affecting photon energy rather than photon number. A fourth common error: leaving out the negative y-intercept meaning on graph questions, which loses an easy mark.

It appears on both. Paper 1 tests it through multiple-choice questions on graphs and the photoelectric equation, while Paper 2 tends to ask for a full explanation using command terms like "explain" or "outline," often paired with a calculation using $hf=\varphi +E_{k(max)}$.

The core photoelectric effect content — the equation, the graph, the explanation — is identical at SL and HL. What changes is the surrounding topic: HL students also cover matter waves and photon momentum, which adds context questions but doesn't change how the photoelectric effect itself is assessed.

It's usually not a maths problem — it's a conceptual one. Students who've spent years thinking of light as a wave find it genuinely hard to switch to describing it as discrete photons, and most exam mark loss comes from blending the two models together rather than from calculation errors.

Past paper questions with official mark schemes are the single best resource, since they show exactly which phrases earn marks on "explain" questions. Alongside that, worked graph-interpretation practice and a clear set of revision notes covering the equation, graph, and common misconceptions cover most of what's examined.