# Photon

A packet of light with energy E = h f = h c / lambda, where h = 6.63e-34 J s.

Physics (university) · Part 10: Relativity and quantum physics · Card 5 of 11 · Last updated 2026-10-19

## Analogy

Light arrives in small bundles, like single drops of rain.

## Example

**Green photon**

E = 1240 eV nm / lambda
= 1240 / 550
= 2.25 eV

## In practice

A green photon of wavelength 550 nm carries about 2.25 eV.

## Practice

### Flashcards

- **What is the energy of a photon?** E = hf = hc/lambda.
- **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

### Is light a wave or a particle?

It behaves as both, depending on the experiment. Interference and diffraction show wave behaviour. The photoelectric effect and single-photon detectors show particle behaviour. Quantum theory treats light as one thing that has both aspects. Physicists call this wave-particle duality, and it also applies to electrons and atoms.

### What is an electron-volt for photons?

The electron-volt is a convenient energy unit for atoms, equal to 1.6e-19 J. A useful formula is E in eV equals 1240 divided by wavelength in nanometres. Visible photons have 1.8 to 3.1 eV, while X-rays carry thousands of eV each.

## Related concepts

- [Photoelectric effect](https://studyvisually.com/learn/science/physics-university/physics-university-part-10/physics-university-part-10-concept-6)
- [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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