21 cm is the wavelength of the 21-cm-line if it was coming from a distance of 8,000,000,000 (8*10^9) light-years
Only when relative velocity changes do wavelength alter. Thus, the wavelength will not change. A waveform signal that is carried in space or down a wire has a wavelength, which is the separation between two identical places (adjacent crests) in consecutive cycles.
This length is typically defined in wireless systems in meters (m), centimeters (cm), or millimeters (mm). The relationship between frequency and wavelength is inversely proportional. The wavelength of the wave with the highest frequency is the shortest. Half the wavelength corresponds to twice the frequency. The wavelength in the red spectrum is the longest.
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