# Photoelectric effect

Light above a threshold frequency knocks electrons out of a metal. Maximum kinetic energy = photon energy minus the work function.

Physics (university) · Part 10: Relativity and quantum physics · Card 6 of 11 · Last updated 2026-10-19

## Analogy

A coin toss from a well: you need a minimum throw before anything escapes.

## Example

**Knocking electrons out**

KE max = hf - work function
= 3.1 - 2.0
= 1.1 eV

## In practice

400 nm light (3.1 eV) on metal with a 2.0 eV work function gives electrons up to 1.1 eV.

## Practice

### Flashcards

- **What does the photoelectric effect show?** Light behaves as photons. Energy depends on frequency, not brightness.

### Interview questions

**Explain the photoelectric effect and why it could not be explained by wave theory.**

- Threshold frequency exists
- Brightness does not change electron energy
- Photons carry energy hf

## Questions people ask

### Why does brightness not matter below threshold?

Each electron absorbs a single photon in one go. If one photon lacks the energy to free it, a thousand more photons will not help, since electrons do not store energy over time. Brightness changes how many electrons are freed, not their individual energies.

### What is the work function?

The work function is the minimum energy needed to free an electron from a metal's surface. It differs by metal: about 2.3 eV for sodium and 4.7 eV for copper. A photon must carry at least this energy to eject an electron.

## Related concepts

- [Photon](https://studyvisually.com/learn/science/physics-university/physics-university-part-10/physics-university-part-10-concept-5)
- [de Broglie wavelength](https://studyvisually.com/learn/science/physics-university/physics-university-part-10/physics-university-part-10-concept-7)
- [Uncertainty principle](https://studyvisually.com/learn/science/physics-university/physics-university-part-10/physics-university-part-10-concept-8)
- [Wave function](https://studyvisually.com/learn/science/physics-university/physics-university-part-10/physics-university-part-10-concept-9)

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