Electromagnetic radiation is energy carried by the electromagnetic field. In quantum theory, it is quantized into [[Photon|photons]]; in many situations it is also well-described as a wave.

## What it includes
The electromagnetic spectrum spans:
- radio
- microwaves
- infrared
- visible light
- ultraviolet
- X-rays
- gamma rays

These are the same phenomenon at different frequencies/energies.

## Production in atoms
A common source is atomic electron transitions:
- An excited electron drops to a lower energy level.
- The atom emits a photon with energy \(E = h\nu\).

For a single emission event, one photon is emitted with one momentum direction. Across many random events, emission can appear in many directions (often approximately isotropic unless conditions impose directionality).

## Wave and particle picture
- Wave picture: propagation, interference, diffraction, polarization.
- Particle picture: discrete absorption/emission events and quantized energy transfer.

Both descriptions are needed for a complete quantum account.

## Cosmology note
The [[Cosmic Microwave Background Radiation]] is relic electromagnetic radiation from the early universe, now observed mainly in the microwave band because of cosmic expansion.

## See also
- [[Photon]]
- [[Electron|Electrons]]
- [[Neutrinos]]
- [[The Big Bang]]
- [[expansion of the universe]]