The principle of phase contrast microscope is based on slight variations in refractive index.
While the rays pass from the light source through the specimen, their velocity may be altered because of the differences in the thickness and physical properties of various portions of the specimen.
A phase contrast microscope is provided with special condensers which contain annular or ring-shaped diaphragms. The diaphragm permits a ring of light to pass through the condenser focussing light on the specimen and a ring shaped diffraction plate in the objective lens. So, the diffracted and undiffracted rays are then brought into phase with each other to produce the image that meets the eye.