The ones you are most likely to come into contact with are the “Bi” versions of CONVERGING and DIVERGING lenses. CONVERGING lenses will direct light rays inwards to a set point, we will name this soon. DIVERGING lenses will direct light rays outwards.



CONVERGING (CONVEX) lenses will bring light rays together at the FOCAL POINT on the OPPOSITE SIDE to their entrance along the PRINCIPAL AXIS.


As you might have guessed, in the case of the CONVEX lens shown there will be a focal point on BOTH sides of the lens, at exactly ONE focal length from the centre point  of the lens either side.


There are THREE rules to remember when dealing with CONVEX lenses:



1. A light ray passing along the principal axis and entering the lens on one side will pass through the focal point on the other.


2. A light ray passing through the focal point and entering the lens on one side will emerge parallel to the principal axis on the other side (essentially the reverse of step 1)


3. A light ray passing through the centre of the lens will continue unaffected as it emerges from the other side.