Hello all, I am not sure where to post this topic so if the moderators feel it is in the wrong section, please relocate to the appropriate section. Light, a generalised term we use for electromagnetic radiation. Light can be classed in two categories: 1)Visible light This is light you can see with your eyes that has a wave-length between 400nm-700nm. More simply the colours you see. (Do not worry I will explain wave-lengths later) 2)Detectable light This is light that can be detected by device that our eyes cannot see, such as the infra-red range of light that is beyond 700nm. In understanding light, I feel it is firstly important to understand what we call the inverse square law. You can read more here: https://en.wikipedia.org/wiki/Inverse-square_law What this means in simple terms , is that the further away an observer is from a light source, the weaker the light becomes, it becomes more stretched and a longer wave-length. In science terms we would state there is less intensity and magnitude of the light. In the link I provided you will notice a diagram on the right of the page that shows you in pictorial form how the inverse square law works. Earlier on I said I would explain wave-length and this I shall do now. At the narrow end of the diagram the light has a shorter wave-length, the light being much more compressed in less space. At the wider end of the diagram the light will have a longer wave-length being stretched out to cover more space. In colours, just remember purple is the shortest visible wave-length and red is the longest visible wave-length, the colours of the rainbow filling between. Any questions about this?