Thick in the middle, or at the edges

The idea

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

Two lenses that look alike

Like mirrors, lenses can be convex or concave. Two lenses look alike from the front.

How could you tell which kind each one is?

• Compare how thick each is at the middle and at the edges
• Compare their colours
• See which one is heavier

Sort the lenses

Each lens is shown cut across, with its thickness marked at the middle and at the edge. Sort it as convex or concave. Hint: convex means bulging outwards, like a convex mirror.

Middle or edges

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

A lens may even have one flat side. What decides its kind is where it is thicker.

• Lens is thicker at | Kind of lens
• The middle | Convex lens
• The edges | Concave lens

Notes

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

Check yourself

This lens is cut across. What kind of lens is it?

A lens is thicker at the edges than at the middle. It is a…

A magnifying glass is thicker at the middle than at the edges. What kind of lens is it?

• A concave lens — correct. Yes. It is thicker at the edges than at the middle.
• A convex lens. A convex lens is thicker at the middle. This one is thin at the middle.
• Not a lens, because one side is flat. A lens can have one flat side. It is clear, with a curved surface, so it is a lens.
• concave lens — correct. Yes. Thicker at the edges means concave.
• convex lens. A convex lens is thicker at the middle.
• flat glass plate. A flat glass plate is the same thickness everywhere.
• A concave lens. A concave lens is thicker at the edges.
• A convex lens — correct. Yes. Thicker at the middle means convex.
• A concave mirror. Light passes through a magnifying glass. It is a lens, not a mirror.