What are the differences between phase contrast microscope and ordinary microscope?
Phase contrast microscope is a kind of 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 the light passes through a relatively transparent specimen, the wavelength (color) and amplitude (brightness) of the light have no obvious changes. Therefore, it is often difficult to distinguish the morphology and internal structure of unstained specimens (such as living cells) by ordinary optical microscope. However, due to the different refractive index and thickness of each part of the cell, the optical path of direct light and diffracted light will be different when light passes through this specimen. With the increase or decrease of optical path, the phase of accelerating or lagging light waves will change (produce phase difference). The phase difference of light can't be felt by the naked eye, but the phase difference microscope can change the phase difference of light into the amplitude difference (light-dark difference) that can be perceived by the human eye through its special devices-annular diaphragm and phase plate, so that the original transparent object shows obvious light-dark difference, and the contrast is enhanced, so that we can clearly observe the living cells and some subtle structures in the cells that can't be seen or seen under ordinary optical microscope and dark field microscope.
Imaging principle of phase contrast microscope: During microscopic inspection, the light source can only pass through the transparent ring of annular diaphragm and be condensed into a light beam after passing through the condenser. When this light beam passes through the inspected object, it will be deflected (diffracted) in different degrees due to the different optical paths of each part. Because the image formed by the transparent ring just falls on the back focus plane of the objective lens and coincides with the conjugate plane on the phase plate. Therefore, the direct light without deflection passes through the conjugate surface, while the diffracted light with deflection passes through the compensation surface. Because of the different properties of the conjugate surface and the compensation surface on the phase plate, they will respectively produce a certain phase difference and weaken the intensity of the light passing through these two parts, and then the two groups of light will converge through the rear lens and travel on the same optical path again, so that the direct light and the diffracted light will interfere with each other and change the phase difference into the amplitude difference. In this way, when the phase difference microscope is used for microscopic examination, the phase difference that cannot be distinguished by human eyes is converted into the amplitude difference (light-dark difference) that can be distinguished by human eyes through the colorless transparent body light.
