Light is made up of electromagnetic particles that travel in waves. The shorter the wavelength; the higher the energy. Every wavelength is represented by a different color, and is grouped into categories (gamma rays, x-rays, ultraviolet (UV) rays, visible light, infrared light, and radio waves) which makes up the electromagnetic spectrum.
The human eye is sensitive (visually receptive) to only visible light on the spectrum. Blue light has a very short wavelength, and so produces a higher amount of energy. Essentially 100% of blue light travels right through our eye and to our retina as it’s not filtered out like all the other colors in the spectrum. This is the problem and this is what PURP can significantly help with.