What is a phase contrast microscope
A phase contrast microscope is a microscope that can convert the phase difference (or optical path difference) generated when light passes through an object into a change in amplitude (light intensity). Mainly used for observing live cells, unstained tissue sections, or stained specimens lacking contrast.
The human eye can only distinguish changes in the wavelength (color) and amplitude of visible light, but cannot distinguish changes in phase. Most biological specimens are highly transparent, and the amplitude of light waves remains basically unchanged after passing through, with only phase changes.
The phase contrast microscope basically converts the optical path difference of visible light passing through the specimen into amplitude difference, thereby improving the contrast between various structures and making them clear and visible. The light refracts through the specimen, deviating from the original light path and being delayed by 1/4 λ (Wavelength), if further increased or decreased by 1/4 λ， Then the optical path difference becomes 1/2 λ， The interference between the two photosynthetic axes is strengthened, and the amplitude increases or decreases, increasing the contrast.
From a structural perspective, the difference between phase contrast microscopy and ordinary optical microscopy lies in:
1. Annular aperture A diaphragm with a circular opening, installed between the light source and the condenser, to form a hollow cone of light passing through the condenser and focus it on the specimen.
2. The phase difference microscope has added a phase plate coated with magnesium fluoride inside the objective lens to delay the phase of direct or diffracted light by 1/4 λ。 There are two areas on the phase plate, the part where direct light passes through is called the "conjugate surface", and the part where diffracted light passes through is called the "compensation surface". The phase board is divided into two types according to its working effect:
(1) A+phase board: delay direct light by 1/4 λ， After the combination of two sets of light waves, the amplitude of the light waves increases, and the specimen structure becomes brighter than the surrounding medium, forming a bright contrast (also known as negative contrast).
(2) B+phase plate: delaying diffraction light by 1/4 λ， After the two sets of light rays merge, the light waves subtract and the amplitude decreases, resulting in a darker specimen structure than the surrounding medium, forming a dark contrast (also known as positive contrast). An objective with a phase plate is called a phase contrast objective, and the word "Ph" is often used on the outer shell of the objective.
