What are the differences between a phase contrast microscope and an ordinary microscope?
Phase contrast microscope is a special microscope that converts the optical path difference (i.e. phase difference) generated when light passes through the details of transparent specimens into light intensity difference.
When light passes through a relatively transparent specimen, there is no significant change in the wavelength (color) and amplitude (brightness) of the light. Therefore, when observing unstained specimens (such as live cells) under a regular optical microscope, their morphology and internal structure are often difficult to distinguish. However, due to the different refractive indices and thicknesses of various parts of the cell, there will be a difference in the optical path length between direct and diffracted light when light passes through this specimen. As the optical path length increases or decreases, the phase of the accelerating or lagging light waves will change (resulting in a phase difference). The phase difference of light is imperceptible to the naked eye, but a phase contrast microscope can use its special device - a circular aperture and a phase plate - to convert the phase difference of light into an amplitude difference (brightness difference) that can be perceived by the human eye through the interference phenomenon of light, thereby making the originally transparent object exhibit significant differences in brightness and darkness, enhancing contrast, and allowing us to observe live cells and some subtle structures inside cells that cannot be seen or clearly observed under ordinary optical microscopes and dark field microscopes.
The imaging principle of a phase contrast microscope: During inspection, the light source can only pass through a transparent ring with a circular aperture, and after passing through a condenser, it is condensed into a beam of light. When this beam of light passes through the object being inspected, it undergoes varying degrees of deflection (diffraction) due to the different optical paths of each part. Due to the fact that the image formed by the transparent ring coincides with the conjugate plane on the phase plate and the focal plane behind the objective lens. Therefore, the straight light without deviation passes through the conjugate surface, while the diffracted light with deviation passes through the compensating surface. Due to the different properties of the conjugate surface and compensation surface on the phase plate, they will respectively produce a certain phase difference and intensity reduction of the light passing through these two parts. The two sets of light will then converge through the rear lens and travel on the same optical path again, causing interference between the direct light and the diffracted light, turning the phase difference into an amplitude difference. In this way, during phase contrast microscopy, the phase difference that cannot be distinguished by the human eye is converted into an amplitude difference (brightness difference) that can be distinguished by the human eye through the light of a colorless transparent body.
