What is the phase contrast method?
The phase contrast method is an optical technique that renders transparent objects visible by converting phase shifts in light into changes in brightness. Imagine two sets of ripples on a pond: one from a stone, another from a submerged obstacle. The obstacle doesn't stop the ripples but shifts their timing—that's a phase shift. In a microscope, a transparent cell does the same to light. My method uses a phase plate in the back focal plane of the objective to separate the direct light from the diffracted light, then introduces a quarter-wavelength shift between them. Interference then produces contrast proportional to the phase difference. I defined this operationally in my 1934 paper 'On the Measurement of Optical Path Differences.' It is a direct measurement of optical path, not a trick of staining.
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