Question

The point on the principal axis where rays parallel to the principal axis actually meet after refraction, or appear to diverge after refraction, is called the:

Options

• Principal focus

• Optical centre

• Centre of curvature

• Aperture

Answer 1

The principal focus is defined as the point on the principal axis where rays parallel to the principal axis actually meet after refraction (convex lens), or appear to diverge after refraction (concave lens). For a convex lens the focus is real, while for a concave lens it is virtual.