BASIC PRINCIPLE

A compound microscope employs two lenses. The objective has a short focal length, in some cases 1 mm or less, and is placed near the specimen or sample to be observed. This produces an image at some distance above the objective, where the light rays come to a focus. The distance (let’s call it s) between the objective and this image is always greater than the focal length of the objective.
The eyepiece has a longer focal length than the objective. It magnifies the real image produced by the objective. In a typical microscope, illumination can be provided by shining a light upward through the sample if the sample is translucent. Some microscopes allow for light to be shone downward on opaque specimens. Figure 1 is a simplified diagram of a compound microscope showing how the light rays are focused and how the specimen can be illuminated.

Laboratory-grade compound microscopes have two or more objectives, which can be selected by rotating a wheel to which each objective is attached. This provides several different levels of magnification for a given eyepiece. In general, as the focal length of the objective becomes shorter, the magnification of the microscope increases. Some compound microscopes can magnify images up to about 2,500 times. A hobby-grade compound microscope can provide decent image quality at magnifications of up to about 1,000 times.

Fig. 1. Illumination and focusing in a compound optical microscope.