The principle of image conversion of Paul prism is that the light enters from the largest rectangular surface in the prism, passes through the inclined plane for total reflection twice, and then passes through the original incident surface, and the image will flip 180.

Paul prism is an isosceles right-angle prism made of glass block, and its end face faces the right angle. Because light only enters and exits in a normal state, the prism has no dispersion effect. The image passing through Paul's prism will flip 180 and travel in the original direction, that is, the direction of travel will also change 180. Paul prisms are usually used in pairs. As a combination of double Paul prisms, the second prism rotates 90 relative to the first prism.