The photoelectric effect cannot be explained by the wave model of light. Three key pieces of
Quantum Physics - OCR A-Level Physics
- The photoelectric effectThe emission of electrons from a metal surface when electromagnetic radiation of sufficiently high frequencyThe number of complete oscillations passing a point per unit time. Measured in hertz (Hz). is incident on it. cannot be explained by the wave model of light. Three key pieces of evidence:
- 1. Instantaneous emissionIn the photoelectric effect, electrons are emitted immediately when photons above the threshold frequency arrive — no time delay.: even at very low intensities, photoelectrons are emitted immediately (within ~10⁻⁹ s). A wave model predicts a gradual accumulation of energyThe capacity to do work. Measured in joules (J)..
- 2. Threshold frequencyThe number of complete oscillations passing a point per unit time. Measured in hertz (Hz).The minimum frequencyThe number of complete oscillations passing a point per unit time. Measured in hertz (Hz). of incident radiation required to cause photoelectric emission from a particular metal surface.: below f₀, no electrons are emitted regardless of how intense the light is. A wave model predicts that any frequency should eventually deliver enough energyThe capacity to do work. Measured in joules (J)..
- 3. KE_max depends on frequency, not intensityThe powerThe rate of energy transfer. Measured in watts (W). transmitted per unit area perpendicular to the wave direction. Measured in W m⁻². Proportional to amplitude squared.: brighter light ejects more electrons but does not increase their maximum speed. A wave model predicts higher intensityThe powerThe rate of energy transfer. Measured in watts (W). transmitted per unit area perpendicular to the wave direction. Measured in W m⁻². Proportional to amplitude squared. should give more energyThe capacity to do work. Measured in joules (J). to each electron.
- The photonA quantum (discrete packet) of electromagnetic radiation. Its energy is proportional to its frequency. model explains all three: one photonA quantum (discrete packet) of electromagnetic radiation. Its energy is proportional to its frequency. delivers its entire energy hf to one electron in a single interaction. If hf < φ, the photonA quantum (discrete packet) of electromagnetic radiation. Its energy is proportional to its frequency. cannot free the electron.
Examiner Tips and Tricks
- A 6-mark question on this topic expects you to state the observation, explain why the wave model fails, and explain how the photon model succeeds, for at least two of the three observations.
- Structure your answer clearly with numbered points.