Wavelength
The distance (measured in meters, or fractions of a meter) from one peak to the next in a wave. When light with different distributions of wavelengths enters our eye, different colours are perceived.
Representation of waves with longer (top) and shorter (bottom) wavelengths. A wavelength is the measured distance from one peak to the next.
The light wave on the top has longer wavelength and lower energy than the light wave on the bottom.
A common misconception about light is that the wavelengths are somehow coloured. See https://colourliteracy.org/light-rays-not-coloured for clarification of this misconception.
