4. Geometrical Optics and thin lenses

            Lenses are everywhere in this world.  They are used for glasses, in telescopes, in photocopy machines, etc.  It is therefore important for us to understand the basic principles of lenses.  There are two types of lenses, converging and diverging.  Convex lenses are converging, and concave lenses are diverging.  Keep in mind, that as Light rays hit the lens, they will bend towards the fatter part of the lens.  This is the reason that a convex lens is converging.  As the Light rays hit the converging lens, they bend towards the center, which is the thicker part, and the Light rays will cross with each other.