1. Present the physical scene
In your last two lessons you built a normal eye. It bends light mostly at the cornea, fine-focuses with a soft lens, and lands a sharp image right on the retina — the screen at the back. A normal eye can do this for everything from 25 cm (its near point) all the way out to infinity (its far point). Near roti, far hilltop — both sharp.
Now look around your classroom. One friend squints at the blackboard but reads a book fine. Another reads the board easily but holds her book at arm's length to see the words. Your grandfather holds the newspaper far away and still asks for more light.
Three different people. Three different problems. None of them is being careless. Something inside each eye is landing the image in the wrong place.
Here is the idea we chase all lesson. A sharp image needs to form exactly on the retina. If the eye lands it a little in front of the retina, or a little behind the retina, only a blur reaches the screen. Each defect of vision is just one story about where the image went wrong — and each is fixed by adding a lens that nudges the image back onto the retina.
Stop scrolling. Try it in your head before reading on. If a sharp image must land on the retina, how many wrong places are there for it to land? Picture the screen and the two ways to miss it.
There are exactly two wrong places: just in front of the retina, or just behind it. That is the whole map. Everything in this lesson is one of those two misses.
You can now state the puzzle: a defect of vision means the image lands in front of or behind the retina instead of on it — and our job is to say which, why, and how to fix it.