The resolving power of a microscope is determined by various conditions of the optical system
1, the numerical aperture, also known as the mirror mouth rate (or opening rate), abbreviated as N.A., in the objective lens and concentrator are labeled with their numerical aperture, the numerical aperture is the main parameter of the objective lens and concentrator, but also an important indicator to determine their performance. Numerical aperture and microscope performance has a close relationship with the resolving power of the microscope is directly proportional to the depth of focus is inversely proportional to the square root of the brightness of the mirror is directly proportional. The numerical aperture can be expressed by the following formula: N.A = n.sin α 2 where: n - the objective lens and the specimen between the media precipitation rate α - the objective lens of the mirror mouth angle The so-called mirror mouth angle is the optical axis from the objective lens of the object point of the light emitted from the objective lens and the objective lens in front of the lens of the effective diameter of the edge of the angle of the sheet, see Figure 1-5. The mirror mouth angle α is always less than 180°. Because the refractive index of air is 1, so the numerical aperture of the dry objective lens is always less than 1, generally 0.05-0.95; oil-immersed objective lens, such as cedar oil (refractive index of 1.515) immersed, the numerical aperture can be close to 1.5 Although the theoretical limit of the numerical aperture is equal to the refractive index of the immersed medium used, but in practice from the perspective of manufacturing technology of the lens, it is not possible to reach this limit. In practice, it is not possible to reach this limit with the lens manufacturing technology. In general, the large numerical aperture of an oil immersion lens is 1.4 within practical limits. The refractive indices of various media are as follows: 1.0 for air, 1.33 for water, 1.5 for glass, 1.47 for glycerin, and 1.52 for cedar oil.
2, the resolving power D can be expressed by the following formula: D = λ/2N.A. The wavelength of visible light is 0.4-0.7 microns, the average wavelength is 0.55 microns. If the numerical aperture of the objective is 0.65, D = 0.55 microns / 2 × 0.65 = 0.42 microns. This means that the specimen can be observed if it is larger than 0.42 microns, but cannot be seen if it is smaller than 0.42 microns. When an objective lens with a numerical aperture of 1.25 is used, D = 2.20 microns. If the length of the observed object is greater than this value, it can be seen. It can be seen that the smaller the D value, the higher the resolution and the clearer the object. According to the above formula, you can: (1) reduce the wavelength; (2) increase the refractive index; (3) increase the angle of the mirror to improve the resolution. Ultraviolet light as a light source for microscopes and electron microscopes is the use of short light waves to improve the resolution to see smaller objects. The resolving power of the objective lens is closely related to whether the image is clear. Eyepieces do not have this power. The eyepiece only magnifies the image produced by the objective.
3, magnification: microscope magnifies the object, first through the objective lens * second magnification of the image, the eyepiece in the clear vision distance caused by the second magnification of the image. The magnification is the ratio of the size of the image to the size of the original object. Therefore, the magnification of the microscope (V) is equal to the magnification of the objective (V1) and the eyepiece magnification (V2) of the product, that is: V = V1 × V2 Comparison of the calculation method, can be obtained from the following formula M = △ × D F1 F2 F1 = Focal length of the objective, F2 = Focal length of the eyepiece △ = Optical tube length, D = visual distance (= 250 mm) △ = the magnification of the objective, D = the magnification of the eyepiece M = microscope magnification F1 F2 Set △ = 160 mm F1 = 4 mm D = 250 mm F2 = 150 mm, then M = △ × D = 160 × 250 = 40 × 16. 7 = 668 times F1 F2 4 15
4, Depth of Focus: When observing a specimen under a microscope, the object is clear when the focus is on a certain image plane, which is the target plane. In the field of view, in addition to the target plane, you can also see blurred objects above and below the target plane, and the distance between these two surfaces is called the depth of focus. The depth of field of the objective lens and the numerical aperture and magnification are inversely proportional, that is, the larger the numerical aperture and magnification, the smaller the depth of field. Therefore, the adjustment of oil mirror than the adjustment of low-magnification mirror to be more careful, otherwise it is easy to make the object slipped through and can not be found.
