In the case of a BICONCAVE (DIVERGING) lens the principal focus is a point on each side of the lens where divergent rays, if backtraced, appear to have come from. As you might have guessed, there are rules with regard to CONCAVE lenses which are essentially modifications of the rules for CONVEX lenses.



1. A light ray passing along the principal axis and entering the lens on one side will refract and travel along a path in line with the nearside principal focus


2. As with CONVEX lenses, an incident ray passing through the centre of the lens will carry on unaltered and in the same direction.


3. An incident ray passing through the lens in the direction of the far side principal focus will refract to a path parallel to the principal axis (again, effectively the reverse of rule 1).