Radio Distance from one peak of a wave to the next. Wavelength is measured in units of distance. The wavelengths of visible light correspond to ~400-650 nm. Wavelength is an important way to characterize a wave. For light, the shorter the wavelength, the higher the energy of the light wave. emission that originates with a neutral 1H atom. The proton and the electron each have a quantum "spin," which points either "up" or "down." Spins can be parallel (both of them "up" or "down"), or antiparallel
(opposite states). The antiparallel state has slightly less energy than the parallel state, so if an atom in the parallel state changes to antiparallel, a
21-cm radio Discrete bundle of light energy. Light of a given energy (frequency) cannot be broken up indefinitely. Rather for a given frequency it comes in discrete bundles with energy (h = Planck's constant and ν= frequency): It is often useful to think of light as a bunch of particle photons; whereas, is emitted.