Light microscope, resolving power

The principle of an electron microscope is based on the light microscope except that electrons are used instead of light. The resolving power of any microscope is given by Abbe s equation... 

The classical polarizing light microscope as developed 150 years ago is still the most versatile, least expensive analytical instrument in the hands of an experienced microscopist. Its limitations in terms of resolving power, depth of field, and contrast have been reduced in the last decade, in which we have witnessed a revolution in its evolution. Video microscopy has increased contrast electronically, and thereby revealed structures never before seen. With computer enhancement, unheard of resolutions are possible. There are daily developments in the X-ray, holographic, acoustic, confocal laser scanning, and scanning tunneling micro-... 

In a light microscope, the particle must be magnified to a size at least as large as the resolving power of the unaided human eye. The resolving power of microscope equals 1 /d, with... 

The resolution or "resolving power" of a light microscope is usually specified as the minimum distance between two lines or points in the imaged object, at which they will be perceived as separated by the observer. The Rayleigh criterion [42] is extensively used in optical microscopy for determining the resolution of light microscopes. It imposes a resolution limit. The criterion is satisfied, when the centre of the Airy disc for the first object occurs at the first minimum of the Airy disc of the second. This minimum distance r can then be calculated by Equation (3). 

The resolving power of a microscope is related, among other things, to the wavelength of the radiation used. With ordinary white light of effective wavelength 550 nm, the resolving power is around 200 nm. Beyond this, there seems no way of... 

FIG. 1.19 Basic optical principle governing the operation of an optical microscope (a) the geometry on which the resolving power d of a microscope is based (b) detail showing how light from both sources must be intercepted by the lens to become part of the image. 

To understand the resolution of electron microscopes and later diffraction techniques the wave approach is more instructive. The resolving power of electron microscopes is substantially better than that of a light microscope, since the wavelength of the electrons, given by the de Broglie relation... 

Colloidal particles are often too small to permit direct microscopic observation. The resolving power of an optical microscope (i.e. the smallest distance by which two objects may be separated and yet remain distinguishable from each other) is limited mainly by the wavelength A of the light used for illumination. The limit of resolution 8 is given by the expression... 

In the late nineteenth century, as physics progressed rapidly, J. J. Thomson discovered the electron the invention of the electron microscope followed several decades later. Because the wavelength of the electron is shorter than the wavelength of visible light, much smaller objects can be resolved if they are illuminated with electrons. Electron microscopy has a number of practical difficulties, not least of which is the tendency of the electron beam to fry the sample. But ways were found to get around the problems, and after World War II electron microscopy came into its own. New subcellular structures were discovered Holes were seen in the nucleus, and double membranes detected around mitochondria (a cell s power plants). The same cell that looked so simple under a light microscope now looked much different. The same wonder that the early light microscopists felt when they saw the detailed structure of insects was again felt by twentieth-century scientists when they saw the complexities of the cell. 

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