A light microscope (Figs. 1. 3a) consists of a light source, which may be the sun or an artificial light, plus three glass lenses: a condenser lens to focus light on the specimen, an objective lens to form the magnified image, and a projector lens, usually called the eyepiece, to convey the magnified image to the eye. Depending on the focal length of the various lenses and their arrangement, a given magnification is achieved. In bright-field microscopy, the image that reaches the eye consists of the colors of white light less that absorbed by the cell.
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| Figure 1.3. Basic design of light and electron microscopes. |
Most living cells have little color (plant cells are an obvious exception) and are therefore largely transparent to transmitted light. This problem can be overcome by cytochemistry, the use of colored stains to selectively highlight particular structures and organelles. However, many of these compounds are highly toxic and to be effective they often require that the cell or tissue is first subjected to a series of harsh chemical treatments.
A different approach, and one that can be applied to living cells, is the use of phasecontrast microscopy. This relies on the fact that light travels at different speeds through regions of the cell that differ in composition. The phase-contrast microscope converts these differences in refractive index into differences in contrast, and considerably more detail is revealed (Fig. 1.5). Light microscopes come in a number of physical orientations (upright, inverted, etc.) but whatever the orientation of the microscope the optical principles are the same.
source:
Bolsover, S. R., Hyams, J.,Shepard, E. (2004). Cell biology: a short course 2nd Ed. Hoboken: New
Jersey. Willey and sons Inc. Publishing
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