Practice question

Living cells remain transparent because differences in refractive index only shift the phase of transmitted light, not its amplitude. Phase contrast microscopy, developed by Zernike, transforms these invisible phase retardations into visible brightness variations. An annular diaphragm creates a hollow cone illuminating the specimen; diffracted and undiffracted rays are separated at the objective by a phase plate that advances or retards by quarter wavelength. Interference then produces contrast, making organelles, nuclei and bacteria appear dark against a bright background without staining, preserving viability for studying cell division and motility.