# Photon energy

Light comes in tiny packets called photons. A photon's energy is E = hc/λ, where h is Planck's constant, c is the speed of light and λ is the wavelength. Shorter wavelength means more energy.

Chemistry (university) · Part 01: Atomic structure and periodic trends · Card 2 of 10 · Last updated 2026-10-19

## Analogy

Like coins: blue light is a heavy coin, red light is a light one. You can't pay with half a coin.

## Example

**Green light photon**

λ = 500 nm = 5.00×10⁻⁷ m
E = (6.626×10⁻³⁴ × 3.00×10⁸) / 5.00×10⁻⁷
E = 3.98×10⁻¹⁹ J

## In practice

UV light can damage skin because each UV photon carries enough energy to break chemical bonds, while a red photon cannot.

## Practice

### Flashcards

- **What equation links a photon's energy to its wavelength?** E = hc/λ. Shorter wavelength means higher energy.

## Questions people ask

### Does brighter light mean more energy per photon?

No. Brightness depends on how many photons arrive each second. Energy per photon depends only on the wavelength. A dim blue lamp gives fewer photons than a bright red one, but each blue photon still carries more energy than each red photon.

### Why do we use joules per photon and kilojoules per mole?

One photon's energy is tiny, around 10⁻¹⁹ J, so it is awkward to use. Chemists multiply by Avogadro's number, 6.022×10²³, to get the energy of one mole of photons. That gives kilojoules per mole, which can be compared directly with bond energies and reaction enthalpies.

## Related concepts

- [Atomic emission spectrum](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-1/chemistry-university-part-1-concept-1)

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