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Understanding Photoelectric Effect

Key idea: Use the photon model (E = hf) to explain threshold frequency, immediate emission, and the frequency/intensity trends in the photoelectric effect (A Level Physics).

  • GCE A-Level H2 Physics 2027
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Learning objectives

  • Use photon energy and momentum and analyse the photoelectric effect.
  • Apply de Broglie wavelength and wave-particle evidence.
  • Interpret wavefunctions, probability density and superposition.
  • Apply uncertainty and infinite-square-well energy quantisation.
  • Analyse atomic energy levels and emission or absorption spectra.

1. Definitions (Must Know)

A. Photon (quantum of radiation)

A photon is a quantum (packet) of electromagnetic radiation.

Photon energy: E = hf

where h is Planck’s constant and f is the frequency.

2. Key Ideas (What Earns Marks)

  • In the photon model, one photon transfers energy to one electron (energy is not shared among many electrons).
  • Emission happens if the photon energy exceeds the work function: hf ≥ Φ
  • This explains:
    • threshold frequency,
    • immediate emission,
    • Kₘₐₓ depends on frequency,
    • emission rate depends on intensity.
Mark-scheme-safe wording

“Intensity changes the number of photons per second.”
“Frequency changes the energy per photon.”

3. Detailed Explanations

A. The photon model explanation (step-by-step)

  1. Light arrives as photons of energy hf.
  2. A surface electron absorbs one photon’s energy in one interaction.
  3. The electron needs energy Φ to escape the surface (work function).
  4. Any leftover energy becomes kinetic energy of the emitted electron: K = hf-Φ

B. Connecting to the four observations

  1. Threshold frequency: if hf < Φ, no emission can occur.
  2. Immediate emission: energy arrives in one interaction, so no time is needed to accumulate energy.
  3. Kₘₐₓ depends on frequency: increasing f increases hf, so the most energetic electrons have larger Kₘₐₓ.
  4. Rate depends on intensity: higher intensity means more photons per second, so more electrons are emitted per second.

4. Common Mistakes

  • Saying photons “share energy across many electrons” (the model is one photon → one electron).
  • Using intensity to explain the threshold (threshold is about energy per photon).
  • Forgetting that not all emitted electrons have Kₘₐₓ (some lose energy in the metal before escaping).

5. Exam Tips

  • If asked “why is Kₘₐₓ less than hf?”, answer: “some energy is used to overcome the work function Φ”.
  • If asked why not all electrons have Kₘₐₓ: “collisions inside the metal reduce kinetic energy before the electron escapes”.

6. Worked Examples

Modelled example 1

Decide if emission occurs

Core

Problem

A metal has work function Φ = 3.0 × 10⁻¹⁹ J. Light of frequency 4.0 × 10¹⁴ Hz shines on it. Take h = 6.63 × 10⁻³⁴ J s. Does photoelectric emission occur in the one-photon model?
Study the worked solution

  1. Calculate photon energy

    Method

    hf = 2.65 × 10⁻¹⁹ J.

    Reason

    Frequency fixes the energy carried by each incident photon.

    Working

    hf = (6.63 × 10⁻³⁴)(4.0 × 10¹⁴) = 2.65 × 10⁻¹⁹ J

  2. Compare with the threshold energy

    Method

    No emission occurs.

    Reason

    Each photon has less energy than the 3.0 × 10⁻¹⁹ J needed to free an electron.

    Working

    hf < Φ

7. Mind Stretchers

Mind stretcher 1: Explaining an observation mismatchExtension

In an experiment, increasing intensity raises the current but the stopping potential is unchanged. Explain this using the photon model.

Show Answer

Higher intensity means more photons per second, so more electrons are emitted per second (current increases).

But photon energy hf is unchanged (frequency unchanged), so the maximum kinetic energy (and therefore stopping potential) is unchanged.

Continue with the next resource in this course.

Course and syllabus information
Course
GCE A-Level H2 Physics
Edition
GCE A-Level H2 Physics 2027