The darkfield condenser/objective pair illustrated in Figure 1 is a high-numerical aperture arrangement that represents darkfield microscopy in its most sophisticated configuration, which will be discussed in detail below. If no specimen is present and the numerical aperture of the condenser is greater than that of the objective, the oblique rays cross and all such rays will miss entering the objective because of their obliquity. The top lens of a simple Abbe darkfield condenser is spherically concave, allowing light rays emerging from the surface in all azimuths to form an inverted hollow cone of light with an apex centered in the specimen plane. These are ideal candidates for darkfield illumination.ĭarkfield illumination requires blocking out of the central light which ordinarily passes through and around (surrounding) the specimen, allowing only oblique rays from every azimuth to "strike" the specimen mounted on the microscope slide. For instance, many small aquatic organisms have a refractive index ranging from 1.2 to 1.4, resulting in a negligible optical difference from the surrounding aqueous medium. Such objects often have refractive indices very close in value to that of their surroundings and are difficult to image in conventional brightfield microscopy. This principle is applied in darkfield (also called darkground) microscopy, a simple and popular method for making unstained transparent specimens clearly visible. In short, the visibility of the faint star light is enormously enhanced against a dark background. ![]() During a total solar eclipse, the moon moves between the Earth and the sun blocking out the light of the sun and the stars can now be seen even though it is daytime. Yet stars are shining both night and day, but they are invisible during the day because the overwhelming brightness of the sun "blots out" the faint light from the stars, rendering them invisible. Stars can be readily observed at night primarily because of the stark contrast between their faint light and the black sky. ![]() All of us are quite familiar with the appearance and visibility of stars on a dark night, this despite their enormous distances from the Earth.
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