Cambridge O Level Physics · Syllabus 5054 · Waves
Principal Focus
What is Principal Focus?
The principal focus of a converging lens is the point on the principal axis to which a beam of light parallel to that axis converges after passing through the lens; for a diverging lens it is the point from which such a beam appears to diverge.
This definition is part of the Waves chapter in Cambridge O Level Physics.
Principal Focus in context
A converging lens brings a parallel beam together at the principal focus; a diverging lens spreads a parallel beam so that it appears to come from a principal focus on the incident side. The distance from the optical centre to the principal focus is the focal length. Where real refracted rays actually meet, the image is real and can be caught on a screen; where only their backward extensions meet, the image is virtual and cannot. Linear magnification is the ratio of image length to object length.