Spectra

Meet Spectra, a light wave who trades frequency for wavelength while always keeping the same speed.

For any wave, speed equals frequency times wavelength, since each cycle moves the wave forward by one wavelength. For light and every other electromagnetic wave in vacuum, the speed is fixed at c = 299,792,458 metres per second. So higher frequency means shorter wavelength, and the two are locked together: c = fλ.

Example: a 100 MHz FM radio signal has a wavelength of about 3.0 m (computed). Visible light, with wavelengths around 400 to 700 nanometres, has frequencies near 10¹⁴ hertz. Radio, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays are all the same kind of wave, differing only in frequency.

The relation sets the size of antennas, which are often about half a wavelength long, and links the colours of light to frequencies.

In a material like glass or water, light travels slower than c, so its wavelength shrinks while its frequency stays the same.