Practice question

Phase contrast requires annular illumination to separate background light from light scattered by the specimen. A phase annulus, a circular slit located in the condenser front focal plane, produces a hollow cone of rays focused on the sample. The direct hollow cone and the diffracted rays from cellular structures travel different paths. At the objective, a complementary phase plate with a ring retards the direct light by quarter wavelength. Their interference converts small phase differences caused by refractive index variations into amplitude contrast, enabling unstained cells to be visualized with excellent definition without fixation.