Parts of a lens

The idea

The centres of the spheres forming a lens's surfaces are its centres of curvature C1 and C2; the principal axis passes through both, the central point is the optical centre O, and the aperture is its effective diameter.

Naming the parts

To talk about lenses clearly we need names for their parts. A lens looks like a small bulge of glass, but it has a very exact shape.

What do you think each curved surface of a lens is a part of?

• A sphere
• A cube
• A flat sheet

Name the parts

Press Next step to meet the parts of a lens one at a time.

Five names

Each surface is part of a sphere, and the two spheres have centres C₁ and C₂. The line through them is the axis. The middle of the lens is O, and the aperture is its width.

The centres of the spheres that form the surfaces of a lens are its centres of curvature, C₁ and C₂. The imaginary line through both is the principal axis. The central point of the lens is the optical centre O. The effective diameter of the lens outline is its aperture.

Notes

A lens has two centres of curvature C₁, C₂, a principal axis through them, an optical centre O and an aperture, its effective diameter.

Check yourself

Which line passes through the two centres of curvature of a lens?

What is the central point of a lens called?

The effective diameter of the circular outline of a lens is its

How many centres of curvature does a lens with two spherical surfaces have?

• The aperture. The aperture is the effective diameter of the lens outline, not the line through the centres of curvature.
• The optical centre. The optical centre is a single point at the middle of the lens.
• The principal axis — correct. Yes!
• The optical centre — correct. Yes!
• The principal focus. The principal focus is the point where parallel rays meet or seem to come from, not the central point of the lens.
• The pole. The pole is the central point of a spherical mirror, not of a lens.
• focal length. The focal length is the distance between the principal focus and the optical centre.
• aperture — correct. Yes!
• radius of curvature. The radius of curvature is the radius of a sphere that forms a surface of the lens.
• One. Each spherical surface is part of its own sphere, with its own centre, so there are two centres.
• None, lenses are flat. A lens has spherical surfaces, and each spherical surface has a centre of curvature.
• Two, one for each surface — correct. Yes!