1. Introduction: Where Does the Picture Sit?
You hold a magnifying glass over a line of small print in a newspaper. Bring it close — the letters look bigger, standing up the right way. Pull it far back — and suddenly the letters flip upside down and shrink.
Same lens. Same newspaper. But the picture you see — the image — changed completely. It went from big and upright to small and upside-down. And somewhere in between, for one moment, you saw nothing clear at all.
The image is the picture of an object that a lens makes. Sometimes you can catch it on a screen. Sometimes you can only see it by looking through the lens. Where it sits, how big it is, and whether it stands up or hangs upside-down — all of that depends on one thing: how far the object is from the lens.
How do you predict the image without holding the lens in your hand? You draw it. Three carefully chosen rays, a few straight lines — and the answer appears on paper. That drawing is a ray diagram, and it is the whole skill of this topic.
Stop scrolling. Try it in your head before reading on. If moving the newspaper changes the image from upright to upside-down, there must be a special distance where the image "flips." Where do you think that flip happens?