UNDERSTAND THE PRINCIPLE OF A DARKFIELD MICROSCOPE

         Introduction of a darkfield microscope

Microbiology, the branch of science that has so vastly extended and expanded our knowledge of the living world, owes its existence to Antoni Van Leeuwenhoek. In 1673, with the aid of a crude microscope consisting of a biconcave lens enclosed in two metal plates, leeuwenhoek introduced the world to the existence of microbial forms of life. Over the years, microscopes have evolved from the simple single lens instruments of leeuwenhoek, with a magnification of 300x to the present-day electron microscopes capable of magnification greater than 250000x.

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Microscopes are designated as either light or electron microscopes. light microscopes use visible light or ultraviolet rays to illuminate specimens. They include brightfield, darkfield phase contrast instruments and fluorescent. This is similar to the ordinary light microscopes; however, the condenser system is modified so that specimen is not illuminated directly. The condenser directs the light obliquely so that the light is deflected or scattered from the specimen, which then appears bright against a dark background. Living specimens may be observed more readily with a darkfield than with a brightfield microscope.

Principle of the Darkfield Microscope

✔ A darkfield microscope is arranged so that the light source is blocked off, causing light to scatter as it hits the specimen.

✔ This is ideal for making objects with refractive values similar to the background appear bright against a dark background.

✔ When light hits an object, rays are scattered in all azimuths. The design of the darkfield microscope is such that it removes the dispersed light, or zeroth order, so that only the scattered beams hit the sample.

✔ The introduction of a condenser and/or stop below the stage ensures that these light rays will hit the specimen at different angles, rather than as a direct light source above / below the object.

✔ The result is a cone of light where rays are deffracted, reflected and/or refracted off the object, ultimately allowing the individual to view a specimen in the darkfield.