Phase contrast microscopy is a technique used to enhance the contrast of transparent or thinly-sectioned specimens. It works by using a special objective lens and a phase plate to contrast the different refractive indices of the specimen with respect to the surrounding medium.

In a phase contrast microscope, the light source is typically a halogen lamp that is directed through a condenser and a phase plate. The phase plate is a circular disk with a transparent center and a opaque ring that is placed in the light path and rotates the phase of the light waves by a small amount. This creates two slightly offset light beams that pass through the specimen at different angles.

When the light waves pass through the specimen, they are refracted (bent) at different angles depending on the thickness and refractive index of the specimen. This creates an interference pattern that is captured by the phase contrast objective lens, which is specifically designed to detect and amplify the contrast created by the interference pattern.

The resulting image is displayed on the eyepieces or a camera and appears to have enhanced contrast and detail compared to a normal brightfield microscope. Phase contrast microscopy is commonly used for observing biological specimens, such as cells and tissues, and is particularly useful for examining transparent or thinly-sectioned samples.