1. Present the physical scene
Close one eye. Look at your hand. Now look at a wall far away. You did nothing with your fingers. You touched no knob. Yet both — your hand and the far wall — came into sharp focus.
That is strange when you think about it. A camera needs you to turn a ring to focus near or far. Your eye does it by itself, in less than a second, all day long.
Here is the idea. Your eye is a tiny natural optical instrument. An optical instrument is any device that bends light to form an image — like a magnifying glass or a camera. Light from an object enters your eye. The eye bends that light. The light lands on a screen at the back. On that screen, a small picture of the object forms. Your brain reads that picture.
So the eye works like a convex lens (a lens that is fat in the middle and bends light to meet at a point) sitting in front of a screen. The lens forms the image; the screen catches it.
Stop scrolling. Try it in your head before reading on. If the eye works like a convex lens forming an image on a screen, is that image straight up, or upside down?
The answer surprises people: the image on the back screen is real (it is actually formed by light landing there, not just a trick of the mind) and inverted (upside down). A convex lens always flips a real image. So the picture inside your eye is upside down. Your brain quietly turns it the right way up. You never notice.
You can now say what the eye is: a natural convex-lens-plus-screen system that forms a real, inverted image of whatever you look at.