Phase Contrast Microscope

The phase contrast microscope, invented by Frits Zernike in 1935, is built on the same basic optical principles as an ordinary light microscope but adds one clever modification that lets it reveal detail in specimens that would otherwise look almost featureless. Living cells are mostly transparent, so under a plain bright field microscope they barely absorb any light and show very little contrast between their internal structures. What differs between those structures is not how much light they absorb, but how much they very slightly slow down and shift the phase of the light passing through them - a difference the human eye cannot detect directly. Zernike's solution was to insert a circular 'Phase Plate', etched with a ring-shaped annular pattern, into the condenser lens system. As the resulting hollow cone of light travels through the specimen, direct (undeviated) light passes through the thin region of the phase plate and travels comparatively fast, while light that has been diffracted by dense structures inside the specimen passes through the thick region and travels more slowly; when these two beams recombine, that timing difference is converted into a visible difference in brightness, i.e. contrast. This is exactly what makes phase contr …