Light Lab · Lenses

refraction → focus lens shape → ray bend

Bend to a point

Convex glass. A thicker middle bends outer rays toward a real focus.

A convex lens is thicker in the middle. It bends outer rays more than the center ray, so they meet at a focal point. You can catch that real focus on the card. A concave lens is thinner in the middle and bends the rays apart. They would meet only behind the glass, at a virtual focus. The center ray hits straight on, so it goes straight through.

Drag orbit · rail sets the curve · C convex · D concave

Where you'll see this

An optometrist writes a glasses prescription in diopters. One diopter means a focal length of one meter. A convex lens bends rays together and corrects a farsighted eye. A concave lens spreads them and corrects a nearsighted eye. Contact lenses do the same job in a thinner piece.

A cinematographer picks a lens by its focal length. A short focal length brings the rays together sooner and takes in a wide view. A long one meets them farther away and takes in a tight view.

An astronomer uses a large lens or a curved mirror in a telescope to bring starlight to one focus. A lab biologist uses a microscope. Its objective is a strong convex lens that bends light from a tiny specimen toward a focus.

Common mistakes

A thicker middle does not always make things bigger. A convex lens is thicker in the middle and bends rays together, so a close object can look bigger. A concave lens is thinner in the middle and bends rays apart, so things look smaller.

The focus is not always a spot you can catch on a card. A convex lens can make a real focus in front of the glass. A concave lens makes a virtual focus behind the glass, where the rays never actually meet.

The center ray is not the one that bends most. It hits the lens straight on and goes straight through. Rays farther from the center bend more.

Lens shape