1. Introduction
A microscope is an optical instrument that produces an enlarged image of an object. A light source built into its base illuminates a specimen through a set of lenses to produce a magnified image that is projected onto the retina of the eye or onto an imaging device (Figures 17.1 and 17.2).The objective lens and the eyepiece (or ocular lens) work together to produce the final magnification M of the image such that
Mfinal = Mobjective x Mocular
Two important components are of critical importance in forming the image. These are:
i. The objective lens, which collects the light that is diffracted by the specimen and form a magnified real image at the real intermediate image plane near the oculars or eyepiece
ii. The condenser lens, which focuses light from the illuminator onto a small area of the specimen
The condenser lens gather the diffuse rays from the light source and illuminates the specimen with a small cone of bright light that allow very small part of the specimen to be seen after magnification. The light rays focussed on the specimen by the condenser lens are then collected by the microscope's objective lens. The light rays are brought to focus by the objective lens to form a real, enlarged image of the object within the column of the microscope. The image formed by the objective lens is used as an object by a second lens system, the ocular lens, to form an enlarged and virtual image. A third lens system located in the front part of the eye uses the virtual image produced by the ocular lens as an object to produce a real image on the retina. For an imaging device, the intermediate image is recorded directly or projected as a real image onto a camera.
Figure 17.1: The light microscope