Photoelectric effect
Quantum Physics - OCR A-Level Physics
Key Definition
Photoelectric effect
The emission of electrons from the surface of a metal when electromagnetic radiation of sufficiently high frequency is incident on it.
The emission of electrons from the surface of a metal when electromagnetic radiation of sufficiently high frequency is incident on it.
$$hf = \phi + E_{k(max)}$$
- Key experimental observations that the wave model cannot explain:
- 1. Emission is instantaneous: there is no time delay, even at low intensities. A wave model predicts energyThe capacity to do work. Measured in joules (J). would accumulate gradually.
- 2. Below the 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. f₀, no electrons are emitted regardless of 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.. A wave model predicts any frequencyThe number of complete oscillations passing a point per unit time. Measured in hertz (Hz). should work given enough 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..
- 3. Increasing 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. increases the number of photoelectrons per second but does NOT increase their maximum kinetic energyThe capacity to do work. Measured in joules (J).The energyThe capacity to do work. Measured in joules (J). an object possesses due to its motion..
- 4. Increasing frequency above f₀ increases the maximum kinetic energyThe energy an object possesses due to its motion. of emitted electrons.
- The photon modelThe model of light as a stream of photons, each carrying energy E = hf. Explains the photoelectric effect. explains all observations: one photonA quantum (discrete packet) of electromagnetic radiation. Its energy is proportional to its frequency. interacts with one electron in a single, instantaneous one-to-one process.
Key Definition
Work function (φ)
The minimum energy required to release an electron from the surface of a metal. Different metals have different work functions. Measured in J or eV.
The minimum energy required to release an electron from the surface of a metal. Different metals have different work functions. Measured in J or eV.
Key Definition
Threshold frequency (f₀)
The minimum frequency of incident radiation required to cause photoemission from a particular metal surface. At this frequency, the photon energy exactly equals the work function: hf₀ = φ.
The minimum frequency of incident radiation required to cause photoemission from a particular metal surface. At this frequency, the photon energy exactly equals the work function: hf₀ = φ.
Common Mistake
MEDIUM
Students often: Avoid saying that increasing intensity increases the kinetic energyThe energy an object possesses due to its motion. of photoelectrons.
Instead: Increasing intensity increases the NUMBER of photons per second, so more electrons are emitted per second (higher photocurrent). The maximum KE depends only on the frequency of the incident photons.
Instead: Increasing intensity increases the NUMBER of photons per second, so more electrons are emitted per second (higher photocurrent). The maximum KE depends only on the frequency of the incident photons.
Examiner Tips and Tricks
When explaining why the photoelectric effectThe emission of electrons from a metal surface when electromagnetic radiation of sufficiently high frequency is incident on it. supports the particle model, you must make three points: (1) one photonA quantum (discrete packet) of electromagnetic radiation. Its energy is proportional to its frequency. interacts with one electron, (2) if hf < φ the photonA quantum (discrete packet) of electromagnetic radiation. Its energy is proportional to its frequency. has insufficient energy and no emission occurs regardless of intensity, (3) emission is instantaneous because energy transfer occurs in a single quantum.