The range of wavelengths over which electromagnetic radiation extends. The longest waves (105-10-3 metres) are radio waves, the next longest (10-3-10-6 m) are infrared waves, then comes the narrow band (4-7 × 10-7 m) of visible light, followed by ultraviolet waves (10-7-10-9 m), X-rays (10-9-10-11 m), and gamma rays (10-11-10-14 m).
- anthropic principle
- antiatom
- antiferromagnetism
- antilogarithm
- antimatter
- antinode
- antiparallel spins
- antiparallel vectors
- antiparticle
- anyon
- aperture
- aperture synthesis
- aphelion
- aplanatic lens
- apochromatic lens
- apocynthion
- apogee
- apolune
- apparent expansivity
- Appleton layer
- applications software
- approximation technique
- apsides
- aqueous
- arc lamp