Porro Prism, also known as Porro prism, is a refractive triangular prism used in optical instruments to modify the orientation of images. Paul prisms are isosceles right-angled prisms shaped from glass blocks with the end planes facing the right angle. In use, the light enters from the largest rectangular surface in the triangular prism, undergoes two total reflections on the inclined surface, and then penetrates the original incident plane and exits. Because the light only enters and exits in a normal state, the prism does not have the effect of dispersion. But the image passing through the Paul prism will be flipped 180°, and will travel in the original direction, that is, the direction of Paul prism travel is also changed by 180°. But because the image is reflected twice, the handedness is unchanged.