Basic optical principles of microscopes
(1) Refraction and refractive index
Light propagates in a straight line between two points in a uniform isotropic medium. When it passes through transparent objects with different density media, refraction occurs. This is due to the different propagation speeds of light in different media. When light rays that are not perpendicular to the surface of a transparent object are incident from the air into a transparent object (such as glass), the light ray changes direction at its interface and forms a refraction angle with the normal.
(2) Lens performance
Lens is the most basic optical element that makes up the optical system of a microscope. Components such as objective lenses, eyepieces, and condensers are composed of single or multiple lenses. According to their different shapes, they can be divided into two categories: convex lenses (positive lenses) and concave lenses (negative lenses).
When a beam of light rays parallel to the optical axis passes through a convex lens and intersects at a point, this point is called the "focus", and the plane that passes through the intersection point and is perpendicular to the optical axis is called the "focal plane". There are two focal points. The focus in the object-space space is called the "object-space focus", and the focal plane there is called the "object-space focal plane"; conversely, the focus in the image-space space is called the "image-space focus". The focal plane at is called the "image square focal plane".
After light passes through a concave lens, it forms an upright virtual image, while a convex lens forms an upright real image. Real images can appear on the screen, but virtual images cannot.
(3) Five imaging rules of convex lenses
1. When the object is located beyond twice the focal length on the object side of the lens, a reduced inverted real image is formed within twice the focal length on the image side and outside the focus;
2. When the object is located at twice the focal length of the object side of the lens, an inverted real image of the same size is formed at twice the focal length of the image side;
3. When the object is located within two times the focal length of the object side of the lens but outside the focus, an enlarged inverted real image will be formed beyond two times the focal length of the image side;
4. When the object is located at the object side focus of the lens, the image side cannot form an image;
5. When the object is located within the focus of the object side of the lens, no image is formed on the image side, and an enlarged upright virtual image is formed on the same side of the object side of the lens at a position farther than the object.
