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Quantum Photoelectric & Line Spectra Explorer

Move between photoelectric observations, stopping-potential graphs, and spectral-line transitions to keep quantum evidence and equations aligned.

  • GCE A-Level H2 Physics 2027
Learning goals
  • Use photon energy and momentum and analyse the photoelectric effect.
  • Analyse atomic energy levels and emission or absorption spectra.

Interactive stageLive model

Loading the interactive model…The interactive model did not load. Try again reloads this page; the explanation and worked content here are still available.

Move frequency across threshold, then change intensity to see what changes and what stays fixed.

Photoelectric effect

Photon rate: medium

  • Photon
  • Electron
  • Threshold

Adjust frequency first: emission only begins when photon energy reaches the work function.

Emission: yesKEmax: 0.88 eVhf: 2.98 eVVs: 0.88 V

  1. Frequencysets photon energyhf = 2.98 eV
  2. Intensitysets electron rateKEmax unchanged
  3. Surfacesets the thresholdf0 = 5.08 × 10¹⁴ Hz

Change one variable at a time and watch the model respond.

Try this: move frequency below and above threshold, then increase intensity.

Explore

Set up the light and surface
55%

KEmax unchanged when only intensity changes.

f0
7.20 × 10¹⁴ Hz

Threshold f₀: 5.08 × 10¹⁴ Hz

Line spectra use transition energy, not the photoelectric frequency slider.

Results

Live readouts

    Photoelectron current

    55%

    Current appears only when frequency is at or above threshold.

    Current calculation

    KEmax = hf - Φ= 2.98 - 2.10= 0.88 eV

    Try a preset, then compare the animation with the readouts.

    Caesium-like surface emits photoelectrons at the current frequency.

    Practice

    Checkpoint questionsOptional exam-style practice after exploring the model.Try

    Prediction target:

        Answer using the current settings shown above.

        Exam formula box

        E = hf

        hf = Φ + KEmax

        KEmax = hf - Φ when hf > Φ

        eVs = KEmax

        ΔE = hf = hc / λ for line spectra.

        Worked example

        If a caesium-like surface has Φ = 2.10 eV and the photon energy is 2.98 eV, then KEmax = 0.88 eV. The stopping potential is therefore about0.88 V.

        Common misconceptions

        • Intensity does not increase photon energy; frequency does.
        • Bright low-frequency light still cannot eject electrons below threshold.
        • Stopping potential measures maximum kinetic energy per unit charge, not emission rate.
        • Line spectra are not continuous rainbows; they come from discrete energy transitions.

        Exam idea

        Use E = hf and Kmax = hf - Φ. Intensity changes how many photons arrive each second, so above threshold it changes emission rate/current, not the maximum kinetic energy or stopping potential.

        Study lensDistinguish intensity effects from frequency effects in photoelectric emission.14 min activity

        Try this

        Keep the evidence chain tight: observation first, then photon explanation, then equation only where it genuinely adds information.

        Learn to

        • Distinguish intensity effects from frequency effects in photoelectric emission.
        • Use threshold frequency and stopping potential as evidence for photon-energy quantisation.
        • Interpret emission and absorption spectra using the energy-level relation ΔE = hf = hc/λ.

        Exam transfer

        • Photoelectric effect
        • Spectral interpretation

        Governing idea

        Photoelectric emission obeys hf = φ + Kₘₐₓ and eVₛ = Kₘₐₓ; spectral photons satisfy ΔE = hf.

        Model boundary

        Surfaces have a single ideal work function and energy levels are simplified. Contact potentials, surface contamination, and line broadening are omitted.

        Avoid this trap

        Above threshold, greater intensity emits more electrons but does not raise their maximum kinetic energy at fixed frequency.

        How to explore

        Link photoelectric observations, stopping-potential graphs, and line spectra without mixing photon energy with intensity.

        Predict the outcome, change one variable at a time, then interpret the result. Completion records participation only and does not award mastery.

        About this activity

        Move between photoelectric observations, stopping-potential graphs, and spectral-line transitions to keep quantum evidence and equations aligned. A text explanation and no-JavaScript route remain available.

        How this activity affects progress
        Course and syllabus information
        Course
        GCE A-Level H2 Physics
        Edition
        GCE A-Level H2 Physics 2027