Light Wavelength
Light Wavelength is the size of a light wave, which sets the color you see.
Think of it this way
Water waves in a bowl can be short and choppy or long and slow. Short waves pack many crests close together. Long waves space each crest far apart. In light, this spacing — the wavelength — sets the color: short waves make violet, long waves make red.
Explaining light wavelength by grade level
Light moves in waves, like ripples in water. Some waves are short and some are long. Short waves make blue light. Long waves make red light. When a plant sits under blue light, it gets short, fast waves that help it grow.