Microscopy is any technique for producing visible images of structures or details too small to otherwise be seen by the human eye, using a light microscope or other magnification tool. It is often used more specifically as a technique of using a microscope. Microscopy has evolved with the development of microscopes. Hence there are three main branches of microscopy; optical, electron and scanning probe microscopy. Optical and electron microscopy involves the diffraction, reflection, or refraction of radiation incident upon the subject of study, and the subsequent collection of this scattered radiation in order to build up an image. This process may be carried out by wide field irradiation of the object for example standard light microscopy and transmission electron microscopy or by scanning of a fine beam over the sample, for example confocal microscopy and scanning electron microscopy. Scanning probe microscopy involves the interaction of a scanning probe with the surface or object of interest.The development of microscopy revolutionized biology and remains an essential tool in that science, along with many others. The light microscope, so called because it employs visible light to detect small objects, is probably the most well-known and well-used research tool in biology. A light microscope works very much like a refracting telescope, but with some minor differences. In contrast to a telescope, a microscope must gather light from a tiny area of a thin, well-illuminated specimen that is close-by. So the microscope does not need a large objective lens. Instead, the objective lens of a microscope is small and spherical, which means that it has a much shorter focal length on either side. It brings the image of the object into focus at a short distance within the microscope’s tube. A second lens, called an ocular lens or eyepiece, as it is brought to your eye magnifies the image. The other major difference between a telescope and a microscope is that a microscope has a light source and a condenser. The condenser is a lens system that focuses the light from the source onto a tiny, bright spot of the specimen, which is the same area that the objective lens examines. Also unlike a telescope, which has a fixed objective lens and interchangeable eyepieces, microscopes typically have interchangeable objective lenses and fixed eyepieces. By changing the objective lenses, going from relatively flat, low-magnification objectives to rounder, high-magnification objectives, a microscope can bring increasingly smaller areas into view — light gathering is not the primary task of a microscope’s objective lens, as it is a telescope’s. A telescope must gather large amounts of light from a dim, distant object; therefore, it needs a large objective lens to gather as much light as possible and bring it to a bright focus. Because the objective lens is large, it brings the image of the object to a focus at some distance away, which is why telescopes are much longer than microscopes. The eyepiece of the telescope then magnifies that image as it brings it to your eye.

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http://www.ruf.rice.edu/~bioslabs/methods/microscopy/microscopy.html