Basic optical principle of microscope

（1） Refraction and refractive index

In a homogeneous isotropic medium, light travels in a straight line between two points. When it passes through a transparent object with different density media, refraction occurs. This is due to the different propagation speed of light in different media. When the light that is not perpendicular to the surface of the transparent object is shot into the transparent object (such as glass), the light changes its direction at the interface and forms a refraction angle with the normal.

（2） Performance of lens

Lens is the most basic optical element of microscope optical system. The objective, eyepiece and condenser are composed of single and multiple lenses. According to their different shapes, they can be divided into convex lens (positive lens) and concave lens (negative lens).

When a beam of light parallel to the optical axis passes through the convex lens and intersects at a point, this point is called "focus", and the plane passing through the intersection and perpendicular to the optical axis is called "focal plane". There are two focal points. The focal point in the object space is called the "object focus", and the focal plane there is called the "object focal plane"; on the contrary, the focus in the image space is called the "image focus", and the focal plane there is called the "image focal plane".

After passing through a concave lens, light becomes an upright virtual image, while a convex lens becomes an upright real image. The real image can be displayed on the screen, but the virtual image cannot.

（3） Five imaging laws of convex lens

1. When the object is located outside the double focal length of the lens, a reduced inverted real image is formed within the double focal length of the image side and outside the focus;

2. When the object is located on the double focal length of the lens, the inverted real image of the same size is formed on the double focal length of the image side;

3. When the object is located within twice the focal length of the object side of the lens and outside the focal point, an enlarged inverted real image is formed outside the double focal length of the image side;

4. When the object is on the focal point of the lens, the image side cannot be imaged;

5. When the object is located in the focal point of the lens, there is no image formation in the image side, and an enlarged vertical virtual image is formed on the same side of the lens far away from the object.