# Bohr energy levels

Electrons in hydrogen have fixed energies, E = -13.6 eV / n squared. Light is emitted when an electron drops a level.

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

## Analogy

Stairs rather than a ramp: no standing between steps.

## Example

**Hydrogen jump**

E = 13.6(1/4 - 1/9)
= 1.89 eV
lambda = 1240 / 1.89 = 656 nm

## In practice

A drop from n = 3 to n = 2 emits a red photon of 1.89 eV (656 nm).

## Practice

### Flashcards

- **What energy does hydrogen have in level n?** E = -13.6 eV / n^2.

## Questions people ask

### Why do elements give off specific colours?

Electrons can only have certain energy levels in an atom. When one drops from a higher to a lower level, it emits a photon whose energy equals the gap. Each element has different gaps, so its light has a set of sharp lines, like a fingerprint.

### What happens at the ionisation energy?

Hydrogen's ground state is -13.6 eV, so an electron needs 13.6 eV to be freed completely. Any photon with at least this energy can ionise a hydrogen atom, leaving a free proton and electron. Extra energy goes into the electron's kinetic energy.

## Related concepts

- [Photon](https://studyvisually.com/learn/science/physics-university/physics-university-part-10/physics-university-part-10-concept-5)
- [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)

Web page: https://studyvisually.com/learn/science/physics-university/physics-university-part-10/physics-university-part-10-concept-10
Study this card in the app: https://studyvisually.com/app/learn?category=physics-university&card=physics-university-part-10-concept-10