Spark image
Lenses - introduction

A lens is simply a piece of transparent material with one or more curved faces, and the effect that it has on a beam of light depends on this curvature.

The point where a beam of parallel light is brought to a focus by the lens is called the principal focus of the lens and the distance of the principal focus from the centre of the lens is its focal length.
The radius of the spherical surface of the lens is called its radius of curvature measured from within the solid.

Using the real is positive sign convention:
convex lens surfaces have a real and therefore positive radius of curvature
concave lens surfaces have a virtual and therefore negative radius of curvature
a convex lens has a real and therefore positive focal length
a concave lens has a virtual and therefore negative focal length

 
 
 
© Keith Gibbs 2010