Imagine the scene. You are sitting in your living room at home concentrating on the latest episode of your favourite TV soap. Out of the corner of your eye you are aware that the cat has just come into the room from the kitchen. The dull glow from the fire tells you that more coal is needed. Just then, the security light bursts into life outside and the shadow of a figure is thrown up against the curtains. At last! you think, the pizza has arrived!
In day-to-day living we pick up huge amounts of information through our eyes: most of the time the sheer quantity is too much and we selectively attend to some things while ignoring others.
So, how are we able to use our eyes to detect all of these things? You probably have some pretty clear ideas about this question, but let's start by considering the act of seeing as one end of a chain linking a source of illumination to a detector (your eyes).