For a basic introduction to this series of posts go to light part 1.
The spectrum is the range of colours that are within white light. There are many different spectrums for different species, e.g. there are certain birds (such as the peregrine falcon) that can see ultraviolet light, whereas we humans cannot see UV or infrared light. This is all to do with the way we see light; in our eyes, different parts pick up different colours, and humans haven’t adapted to have those parts that see UV or infrared, we simply don’t need to. But birds, especially birds of prey have UV vision because it helps them to pick out their prey a lot easier. As Newton discovered, one of the easiest ways to show all the colours in the visible spectrum is to use a prism. He did this by shining a light at a glass prism: the light was separated by the triangle so that all the colours are separated. To prove this, there have been several experiments where there are two prisms and a light, and you can clearly see the separation and reformation of the colours in the light. Humans can only see what we call the visible spectrum, but there are many more colours out there that are undetectable to the human eye. There are only certain things that reflect ultraviolet light, and often can only be seen under UV.