This topic is about one number that an optician writes on your eyeglass prescription: the power of a lens. It is not magnification. It is not focal length by itself. It is a clean measure of how strongly a lens bends light. The whole lesson is about making that number physically real — what it measures, what its unit means, and the one confusion that wrecks most students: forgetting to turn the focal length into metres first.
1. Hook — the strong lens and the weak lens
Picture two reading glasses on a shop counter. Both are convex lenses. You hold each one over a line of small print in a newspaper.
One barely changes the letters. The other blows them up a lot, and you can see the letters bulge as you tilt it.
Hold that picture. Two lenses, same shape family, but one bends the light far more than the other.
Stop scrolling. Try it in your head before reading on — which lens do you think is the "stronger" one, and what about it makes it stronger?