Sunlight has a range of wavelengths, and these have been classified into four main groups: ultraviolet, visible, near infrared and far infrared. We can see the light in the visible spectrum (red, orange, yellow, green, blue indigo, violet) but we can’t see ultraviolet light and neither can we see infrared light.
Much research has been done on the effect of the sun’s ultraviolet light. It can cause premature ageing and increases the risk of skin cancers, hence the warnings to wear sunscreen to block those wavelengths from hitting the skin.
Until recent years, the effect of red and near infrared light, had been of less interest to medical science. It is now an area of increasing research, and the implications seem to be huge.