phase contrast microscope

A microscope that enables living, and hence normally transparent, material to be viewed by converting (invisible) differences of phase in the transmitted light to (visible) differences of contrast. Light passing through a specimen changes its phase by different amounts due to differences in thickness or refractive index. The phase contrast microscope converts these differences to differences in amplitude, which can be seen as shades of grey (because the greater the amplitude of a wave the more bright or intense is the light). The effect is produced by using a special condenser together with a glass plate (the retardation plate) in the objective. The condenser produces a certain pattern of light, often a hollow cone. This is matched in the retardation plate by a similar pattern produced by a groove in the plate. Any light that is not diffracted by the specimen will pass through the groove in the retardation plate and remain unchanged in phase. Any light that has been diffracted by the specimen will pass through the thicker part of the retardation plate and hence be slowed even further. Contrast results from combination of the direct and refracted light. If they are out of phase by half a wavelength the difference produces destructive interference of the light so producing contrast. When this   technique   is   used   in   conjunction with a time-lapse cine camera it enables such events as cell division to be filmed and later speeded up, a technique known as cinemicrography. See also interference microscope.