Human eyes, like the eyes of the leopard's prey, become useless in low light. Until recently, this meant that we had no way of observing the night behavior of leopards. Today, infrared-sensitive cameras enable us to record on videotape what we are unable to see with our eyes. In order to understand how this is possible in total darkness, we must first understand light. Light travels in waves, much like ripples on a pool.
Colors are simply light waves of different lengths.
The waves can even be measured. Looking at a chart, one quickly realizes that the wavelengths we can see make up a very small portion of the total spectrum. As they get shorter than indigo, they become ultraviolet waves, X rays, and finally gamma waves. These waves surround us all the time, but we are unaware of their presence, since they are not in our visible light spectrum. Near-infrared waves behave much like the light waves that we can see.