Together or apart, through glass

The idea

A light beam passes through a thin glass plate unchanged, but a convex lens converges light (a converging lens) and a concave lens diverges it (a diverging lens).

A beam through a lens

A beam of parallel rays passes through a convex lens.

What do you think happens to the rays?

• They come closer together
• They stay parallel
• They spread out

Through three pieces of glass

Send the same beam through a thin glass plate, a convex lens and a concave lens. Watch the rays on the far side.

Converging and diverging

A light beam passes through a thin glass plate as it is. A convex lens converges light: it is a converging lens. A concave lens diverges light: it is a diverging lens.

Careful: a concave mirror converges light, but a concave lens diverges it.

• Beam passes through | What happens
• Thin glass plate | Passes as it is
• Convex lens | Converges: converging lens
• Concave lens | Diverges: diverging lens

Notes

A beam passes through a thin glass plate as it is. A convex lens converges light (converging lens); a concave lens diverges it (diverging lens).

Check yourself

Why is a convex lens also called a converging lens?

A beam of light passes through a thin glass plate. What happens to it?

A concave lens is also called…

Which of these converges a beam of light, just as a convex lens does?

• It spreads light rays out. That is a concave lens: a diverging lens.
• It makes light rays come closer together — correct. Yes. It converges the light falling on it.
• It sends light back. A lens lets light pass through. A mirror sends it back.
• It passes through as it is — correct. Yes. Its surfaces are flat, so the beam does not converge or diverge.
• It converges. A convex lens does that. A flat glass plate does not.
• It diverges. A concave lens does that. A flat glass plate does not.
• a concave mirror. A mirror reflects light. A concave lens lets light pass through and spreads it out.
• a converging lens. That is the convex lens.
• a diverging lens — correct. Yes. It spreads light out.
• A convex mirror. A convex mirror diverges the beam it reflects.
• A concave lens. A concave lens diverges light.
• A concave mirror — correct. Yes. A concave mirror converges the beam it reflects.