What is infrared light?
Infrared, visible, and ultraviolet light are all electromagnetic waves. However these waves differ in both their wavelengths (the distances between adjacent maximums in their electric fields) and in their frequencies (the number of electric field maximums that pass by a specific point in space each second).
Infrared light has longer wavelengths and lower frequencies than visible light, while ultraviolet light has shorter wavelengths and higher frequencies than visible light. We can’t see infrared or ultraviolet lights because the cells of retinas aren’t sensitive to these lights.
Nonetheless, we can often tell when those lights are present–we may feel infrared light as heat on our skins and we may find ourselves sunburned by ultraviolet light.