Let's start with an "eye." This drawing represents a normal human eye, with the retina, where light rays enter, right in the front-center. We'll start our ray-tracing analogy with the human eye, and then expand it to include how we actually ray trace.

First of all, we'll get some facts about the eye straight. All light comes in through the retina - the "hole" in front of the eye, which we should treat (mathematically) as a point. It then travels through the eye and hits the back, where the detectors are. In fact, you'll see shortly that the image that hits the back of the eye is "reversed," and that the brain does the calculations to put it right-side up.