The phase contrast microscope allows unmarked cells to be observed and is especially useful for living cells. Most living organisms can not be stained because the colorants used can damage their cell structure to the point of death . This microscopy technique takes advantage of small differences in refractive indices in different parts of a cell and in different parts of a tissue sample. Light passing through regions of higher refractive index experiences a deflection and is out of phase with respect to the main beam of light passing through the sample. It outputs other out-of-phase wavelengths by means of a series of optical rings of the target and the capacitor, cancels out the amplitude of the out-of-phase portion of the light beam and produces a useful contrast over the image. The dark parts of the image correspond to the dense portions of the specimen; the clear parts of the image correspond to less dense portions. Therefore these microscopes are used to observe living cells, living tissues and semifinished cuts not colored.
Two modifications of the phase microscope are the interference microscope and the differential interference microscope. Its inventor was in 1932 the Dutch physicist Frits Zernike, which together with the method of phase contrast earned him to win the Nobel Prize in Physics in 1953. Bibliography
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