# Electromagnetic spectrum

The family of EM waves ordered by frequency: radio, microwaves, infrared, visible, ultraviolet, X-rays, gamma rays.

Physics (university) · Part 09: Induction, light and optics · Card 7 of 11 · Last updated 2026-10-19

## Analogy

One piano keyboard, with visible light as a few keys.

## Example

**Spectrum order**

Radio: longest wavelength
Visible: 400 to 700 nm
Gamma: shortest

## In practice

Visible light has wavelengths of about 400 nm (violet) to 700 nm (red).

## Practice

### Flashcards

- **Which EM waves have the highest frequency?** Gamma rays.

## Questions people ask

### What is the difference between radio waves and X-rays?

They are the same kind of wave with different frequency. Radio waves have long wavelengths and low energy, so they pass through bodies. X-rays have wavelengths around a nanometre or less, with enough photon energy to pass through soft tissue and ionise atoms.

### How is photon energy related to frequency?

Each photon carries energy E = h f, where h is Planck's constant, 6.63e-34 J s. Higher frequency means more energy per photon. A violet photon at 7.5e14 Hz has about 5e-19 J, or around 3 eV, more than a red photon.

## Related concepts

- [Electromagnetic wave](https://studyvisually.com/learn/science/physics-university/physics-university-part-9/physics-university-part-9-concept-6)
- [Snell's law](https://studyvisually.com/learn/science/physics-university/physics-university-part-9/physics-university-part-9-concept-8)
- [Total internal reflection](https://studyvisually.com/learn/science/physics-university/physics-university-part-9/physics-university-part-9-concept-9)
- [Thin lens equation](https://studyvisually.com/learn/science/physics-university/physics-university-part-9/physics-university-part-9-concept-10)

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