In phase contrast microscopy, what is used to generate a hollow cone of light?
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 visualize
Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.