1. Introduction: The Glass That Catches the Sun
Have you ever taken a magnifying glass into the afternoon sun? You hold it over a scrap of paper. You move it up and down until the bright spot of sunlight shrinks to one tiny, dazzling dot. Hold it still — and the paper starts to smoke.
Stop scrolling. Try it in your head before reading on. What did the glass do to all that sunlight to make one small spot so hot?
It gathered light. Sunlight was falling on the whole face of the glass — a wide circle of light. The glass bent every one of those rays and brought them together to a single point. All that energy, squeezed onto one dot. That is why it burns.
That gathering glass is a convex lens. In this lesson you will meet it and its opposite twin, the concave lens — the one that spreads light out instead of gathering it. You will learn the exact words physicists use to describe a lens, so that everything you do later — drawing ray diagrams, using the lens formula — rests on names you actually understand.
This lesson is about WHAT a lens is and the names for its parts. We are not yet drawing images or using any formula. We are building the vocabulary first.