Final answer:
Phase-contrast microscopy uses a special phase plate to cause destructive interference, enhancing contrast in transparent specimens without staining. Light undergoes phase changes when passing through different structures within the specimen, and when these altered light waves are combined, they create high-contrast images.
Step-by-step explanation:
Understanding Phase-Contrast Microscopy
Phase-contrast microscopy is a special type of light microscopy that enhances contrast in transparent specimens without the need for staining. A typical phase-contrast microscope uses a light source which is focused using an annular stop to create a hollow cone of light. This light passes through the specimen and into a specially designed objective lens that contains a phase plate. The phase plate is designed such that it alters the phase of light passing directly through it by about half a wavelength, causing destructive interference when the direct light and the light diffracted by the specimen are superimposed in the image plane. As a result, structures in the specimen create a high-contrast image due to the interference patterns of light waves that differ in phase because of their interaction with the sample structures.
Light rays that pass through the object and those that pass through the surrounding medium (background) experience different refractive indices, leading to differences in phase. These phase differences, once the light rays are recombined, cause patterns of constructive and destructive interference, thereby enhancing the contrast and making the transparent structures within the sample visible against the bright background. The principle of phase-contrast microscopy was so groundbreaking that its developer, Frits Zernike, was awarded the Nobel Prize. This technique is highly valued for examining live, unfixed, and unstained specimens.