What is the optical principle of metallographic microscope (bright field, dark field)! ?

The main feature of bright field is based on the color and transmittance of the specimen, and the specimen usually needs dyeing to facilitate observation. Of course, you can also narrow the aperture or the upper and lower condenser. Bright field of view is the basis of all other optical microscopes. Dark field is a method designed according to Tindal effect to observe the detected object under dark background. Under normal circumstances, people can't see the indoor dust, because dust particles are interfered by direct light, diffraction and other factors. However, if the light passes through the window under dark indoor conditions, we can easily see it, because when the light is reflected or diffracted, the size of dust particles seems to increase. Dark field is to prevent direct light from entering the lens of the objective lens by special methods. Instead, the direct light changes the optical path through the dark field condenser, making it obliquely enter the measured object, and the reflected or diffracted light on the surface enters the objective lens, forming a bright image printed on a dark background.