Practice question

Phase contrast contrast generation relies on a specialized phase plate housed in the objective back focal plane. The plate contains a conjugate ring that selectively shifts phase of direct, undiffracted light by quarter wavelength and attenuates its amplitude, while diffracted light from specimen passes outside ring unaltered. When recombined, slight phase differences introduced by transparent cells interfere destructively or constructively, converting to visible amplitude differences. This elegant interferometric design enables unstained amoebae, bacteria and cultured fibroblasts to be examined with dark contours on bright background without chemical contrast enhancement.