10.5 Lenses

Look through a flat windowpane and everything stays its normal size. But curve that transparent surface, and it bends light to enlarge or shrink what you see — that’s a lens.

Activity 10.8 — A water-drop lens Put a small drop of water on a lightly oiled glass strip, lay it over printed text, and look down through the drop. The letters underneath look bigger! The drop’s surface bulges outward, and that curve magnifies the text — the water drop is behaving as a simple lens, just like a magnifying glass.

Two Kinds of Lens A lens is a piece of transparent glass or plastic with curved surfaces. Unlike a mirror, light passes through a lens — we look through a lens, not in it.

A convex lens is thicker in the middle than at the edges.
A concave lens is thicker at the edges than in the middle.

Tell the two apart by their shape:

Images through lenses

What You See Through a Lens Look through the lenses at an object (Activity 10.9):

Convex lens: object close → erect and enlarged; moved away → flips to inverted, first enlarged then diminishing. (Just like a concave mirror!)
Concave lens: the image is always erect and diminished. (Just like a convex mirror!)

Converging vs diverging lenses

Lenses Bend Beams Too Pass parallel beams through them (Activity 10.10): a flat glass plate lets light through unchanged, a convex lens converges the light, and a concave lens diverges it. So a convex lens is a converging lens, a concave lens a diverging lens. And yes — a convex lens can burn paper by focusing sunlight, exactly like a concave mirror (Activity 10.11). (Never look at the Sun through a lens.)

Lenses Everywhere — Including in You Lenses are all around us: spectacles to see clearly, cameras, telescopes and microscopes. Most amazing of all, your eye contains a convex lens that can change its shape — letting you switch focus from a book up close to a mountain far away.