A specimen on the stage of a light microscope.
The advantages and disadvantages of the light microscope relate to light, magnification and resolution. Light microscopes magnify visible light--an obvious advantage, since this is what our eyes can see. Magnification (how large an object appears) and resolution (the clarity of details) are both limited when using light microscopes.
Light Source
A basic light microscope with mirror light source.
Light microscopes use either a reflective mirror or an electric light to direct light through the specimen and into the lens system. Mirror systems are less costly, but require adequate room lighting and more patience to adjust. Electric light systems are more costly and require a nearby outlet, but are simpler to use.
Light Intensity
Light intensity (brightness) is important since the light passes through the specimen you are viewing. Thin, translucent (clear) specimens are best viewed with low-intensity light; while thicker, opaque specimens require brighter light. A disadvantage of light microscopes is that some specimens are too thick or opaque to be viewed at all.
Very thin or translucent specimens can be stained in order to increase contrast for better viewing. However, this process will kill live specimens.
Adjusting Light Intensity
The fixed-aperture diaphragm selector is visible under the stage of this microscope.
The diaphragm, located above the light source and below the stage (specimen platform), adjusts the amount of light passing through the specimen. Two kinds of diaphragms are available: a fixed-aperture selector and a camera-style adjustable aperture.
The fixed-aperture consists of several different-sized openings on a rotating plate. The desired opening is selected by rotating the selector. Fixed-aperture diaphragms are less expensive, but provide less precise control over light intensity.
Adjustable-aperture diaphragms provide continuously variable aperture size, much like the f-stop on a camera lens, and thus provide more precise control over light intensity. These systems are more costly.
Magnification
Bigger is not always better. Light microscopes can magnify objects up to 1000x (one thousand times larger than life) fairly well. Beyond that the image becomes increasingly distorted and blurry. Increased size no longer makes a better image, and in fact makes the image unusable.
Using magnification up to 1000x, all types of living organisms can be seen, down to the smallest bacteria cells. This makes light microscopes a powerful tool for study of cell types, pond water, soil samples and other studies where an overview of microorganisms is desired. Light microscopes, however, are less useful for studying subcellular structures, because of the limits of resolution inherent in the use of light.
Resolution
Resolution is a measure of the clarity of fine detail produced in an image. Low-resolution images appear blurred, or "fuzzy." High-resolution images are crisp, clear and detailed. The biggest disadvantage of light microscopes is their limit of resolution. Beyond about 1000x magnification, light microscopes quickly lose the ability to resolve fine details. This results from the physical properties of light, not from the quality of the instrument. To resolve details of subcellular structures, other technologies such as electron microscopes must be used.
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