Electromagnetic radiation is energy carried by the electromagnetic field. In quantum theory, it is quantized into 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.