Red and Infrared light are part of the spectrum of light emitted by the sun. The sun’s light is actually made up of lots of different colours, or ‘wavelengths’ and only a tiny part of the full spectrum of the sun’s light is visible to the human eye.
Red light sits at the highest end of the visible light spectrum, so you can see it. Infrared light sits just outside the visible light spectrum so you can’t actually see it. These red and infrared wavelengths of light are ‘bioactive’ in humans, meaning that they literally affect the function of our cells. Our skin cells contain receptors for red and infrared light, so they recognise and respond to it.