What are the differences between phase contrast microscopes and ordinary microscopes?
Phase contrast microscopy is a special microscope that converts the optical path difference (ie, phase difference) generated when light passes through details of a transparent specimen into a light intensity difference.
When light passes through a relatively transparent specimen, there is no obvious change in the wavelength (color) and amplitude (brightness) of the light. Therefore, when observing unstained specimens (such as living cells) with an ordinary optical microscope, its morphology and internal structure are often difficult to distinguish. However, due to the differences in refractive index and thickness of various parts of the cell, when light passes through this specimen, the optical path lengths of direct light and diffracted light will be different. As the optical path length increases or decreases, the phase of the light waves that speed up or lag behind changes (creates a phase difference). The phase difference of light cannot be felt by the naked eye, but the phase contrast microscope can use its special device - annular diaphragm and phase plate to use the interference phenomenon of light to convert the phase difference of light into an amplitude difference (light and dark) that can be detected by the human eye. difference), so that the original transparent object shows obvious differences in light and dark, and the contrast is enhanced, allowing us to more clearly observe living cells and intracellular components that cannot be seen or cannot be seen clearly under ordinary optical microscopes and dark field microscopes. certain microstructures.
The imaging principle of phase contrast microscope: The light source for microscopic examination can only pass through the transparent ring of the annular diaphragm, and then be condensed into a light beam by the condenser. When this light beam passes through the object to be inspected, the light will be different due to the different optical path lengths of each part. degree of deflection (diffraction). Because the image formed by the transparent ring happens to coincide with the rear focal plane of the objective lens and the conjugate plane on the phase plate. Therefore, the undeflected direct light passes through the conjugate surface, while the deflected diffracted light passes through the compensation surface. Due to the different properties of the conjugate surface and the compensation surface on the phase plate, they will produce a certain phase difference and intensity weakening in the light passing through these two parts respectively. The two sets of light are then converged by the rear lens and return to the same optical path. Traveling, the direct light and the diffracted light will interfere with each other, changing the phase difference into an amplitude difference. In this way, during phase contrast microscopy, the light passing through the colorless transparent body converts the phase difference that is indistinguishable to the human eye into an amplitude difference (difference between light and darkness) that the human eye can distinguish.
