ABSTRACT
There is one fundamental property of a lens from which many of its other characteristics follow on automatically. A lens, by deŽnition, makes parallel rays of light meet at a point, called the focus of the lens (Figure 1.1). The distance from the lens to the focus is called the focal length. For a simple understanding of the microscope the focal length is the only thing we need to know about a lens. This ignores any imperfections of real lenses, assuming that all lenses are perfect, but it is a great starting point.
