The missing contrast was in the wave
A thin transparent object can mainly delay phase while transmitting nearly the same amplitude. An in-focus intensity image then has little first-order contrast for a weak phase object. Staining creates absorption contrast, but changes specimen preparation and may alter what can be observed alive.
Zernike’s 1953 lecture begins with diffraction-grating imperfections and faint spectral ghosts. Comparing them against a coherent reference exposed their relative phase. The microscope method grew from controlling that phase relation, not from discovering that transparent objects absorb strongly.
Separate the waves before recombining them
The objective forms a source image near its back focal plane. Direct light occupies that image, while specimen-diffracted light is distributed around it. A suitably placed phase strip or ring changes their relative phase and often attenuates the direct component. They overlap again in the image plane.
The reference does not provide a second photograph of the specimen. It supplies a field against which the small specimen-induced change can interfere. Placing a coloured overlay on the final image cannot perform this optical operation.
Sources: [1]
A small-phase calculation shows the mechanism
Choose a real incident field amplitude of one and a small dimensionless specimen phase. With no special optics, the first-order specimen field is imaginary relative to the direct field, so its linear intensity contribution vanishes. If the direct field alone receives a positive quarter-cycle phase shift, the idealized image intensity gains a term linear in specimen phase.
The sign below follows that chosen positive shift; reversing the relative phase reverses the contrast. The calculation neglects absorption, finite numerical aperture and the spatial extent of the phase ring. It illustrates contrast generation, not a quantitative phase-reconstruction algorithm.
Sources: [1]
A bright edge is not a new material boundary
Practical phase contrast has finite pupils and a phase element that also affects some low-angle diffracted light. Halos and shade-off can therefore appear. Brightness is not a universal map of thickness, refractive index or dry mass without a calibrated transfer model.
This limit also separates optical Zernike phase contrast from electron phase-contrast imaging. Both use interference, but their illumination, interaction potentials and transfer functions differ. One article should not certify both instruments from the shared word phase.
Sources: [1]
Locate the reference before interpreting the image
Read the lecture’s grating experiment and phase-strip construction before the biological application. In the linked model, compare amplitude and phase specimens at the same aperture, then change the pupil treatment. The useful prediction is how the image changes when the relative phase changes.
Sources: [1]