3.2.2.1
Threshold frequency and work function define when emission starts
The Photoelectric Effect — AQA A-Level Physics
Key Definition
Threshold frequency — The minimum frequency of incident electromagnetic radiation required to remove a photoelectron from the surface of a metal.
Key Definition
Threshold wavelength — The longest wavelength of incident electromagnetic radiation that would remove a photoelectron from the surface of a metal.
Key Definition
Work function (φ) — The minimum energy required to release a photoelectron from the surface of a metal.
- 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. ($f_0$) and work functionThe minimum energyThe capacity to do work. Measured in joules (J). required to liberate an electron from the surface of a metal. ($\phi$) are related by: $\phi = hf_0$.
- Threshold frequencyThe number of complete oscillations passing a point per unit time. Measured in hertz (Hz).The minimum frequency of incident radiation required to cause photoelectric emission from a particular metal surface. and wavelengthThe minimum distance between two points on a wave that are in phase (e.g. crest to crest). Measured in metres (m). are related by: $f_0 = c / \lambda_0$.
- Both are properties of the metal — different metals have different values.
- A photonA quantum (discrete packet) of electromagnetic radiation. Its energy is proportional to its frequency. must have energyThe capacity to do work. Measured in joules (J). $\geq \phi$ to eject an electron. Below this, no emission occurs 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..
The energyThe capacity to do work. Measured in joules (J). well analogy
- Think of surface electrons as sitting in an energy well. The depth of the well equals the work functionThe minimum energy required to liberate an electron from the surface of a metal. $\phi$.
- A photonA quantum (discrete packet) of electromagnetic radiation. Its energy is proportional to its frequency. with energy $\geq \phi$ lifts the electron out of the well.
- A photonA quantum (discrete packet) of electromagnetic radiation. Its energy is proportional to its frequency. with energy $< \phi$ cannot free the electron, no matter how many such photons arrive.
- Alkali metals (sodium, potassium) have low work functions — visible light can cause emission.
- Transition metals (zinc, iron) have higher work functions — UV light is needed.