The magnification of an optical microscope is the product of the magnification of the objective lens and the magnification of the eyepiece. For example, if the objective lens is 10× and the eyepiece is 10×, the magnification is 10×10=100.

1. Classification of objective lenses:

The objective lens can be divided into dry objective lens and liquid immersion objective lens according to the different conditions of use; among them, the liquid immersion objective lens can be divided into water immersion objective lens and oil immersion objective lens (commonly used magnification is 90-100 times).

According to the different magnifications, it can be divided into low magnification objective lens (less than 10 times), medium magnification objective lens (about 20 times) and high magnification objective lens (40-65 times).

2. The main parameters of the objective lens:

① Magnification refers to the ratio of the size of the image seen by the eyes to the size of the corresponding specimen. It refers to the ratio of lengths rather than the ratio of areas. Example: The magnification factor is 100×, which refers to a specimen with a length of 1 μm. The length of the enlarged image is 100 μm. If it is calculated by area, it is magnified 10,000 times.

②. Numerical aperture is also called aperture ratio, abbreviated as NA or A. It is the main parameter of objective lens and condenser, and it is directly proportional to the resolution of the microscope. Dry objectives have a numerical aperture of 0.05-0.95 and oil immersion objectives (cedar oil) have a numerical aperture of 1.25.

③. Working distance refers to the distance from the bottom of the front lens of the objective lens to the top of the cover glass of the specimen when the observed specimen is the clearest. The working distance of the objective lens is related to the focal length of the objective lens. The longer the focal length of the objective lens, the lower the magnification, and the longer its working distance. Example: 10x objective lens is marked with 10/0.25 and 160/0.17, where 10 is the magnification of the objective lens; 0.25 is the numerical aperture; 160 is the length of the lens barrel (in mm); 0.17 is the standard thickness of the cover glass (in mm). The effective working distance of the 10x objective lens is 6.5mm, and the effective working distance of the 40x objective lens is 0.48mm.

3. The function of the objective lens is to magnify the specimen for the first time. It is the most important part that determines the performance of the microscope——the resolution.

Resolution is also called resolution or resolving power. The size of the resolution is expressed by the value of the resolution distance (the minimum distance between two object points that can be resolved). At the photopic distance (25cm), normal human eyes can clearly see two object points that are 0.073mm apart. The value of 0.073mm is the resolution distance of normal human eyes. The smaller the resolution distance of the microscope, the higher its resolution, and the better its performance.

When using the common central illumination method (the photopic illumination method that allows the light to pass through the specimen evenly), the resolution distance of the microscope is d=0.61λ/NA

NA - the numerical aperture of the objective lens

For example, the numerical aperture of the oil immersion objective lens is 1.25, and the wavelength range of visible light is 400-700nm. If the average wavelength is 550 nm, then d=270 nm, which is about half the wavelength of the illumination light. In general, the limit of resolution of microscopes illuminated with visible light is 0.2 μm.

(2), eyepiece

1. Structure of the eyepiece

2. The role of the eyepiece
It is to further magnify the clearly resolved real image that has been magnified by the objective lens to the extent that the human eye can easily distinguish it clearly. The magnification of commonly used eyepieces is 5-16 times.

3. Relationship between eyepiece and objective lens
The fine structure that has been clearly resolved by the objective lens, if it is not re-magnified by the eyepiece and cannot reach the size that the human eye can distinguish, then it will not be seen clearly; but the fine structure that cannot be resolved by the objective lens can still be clearly seen even though it is re-magnified by a high-power eyepiece, so the eyepiece can only be used for magnification, and will not improve the resolution of the microscope. Sometimes although the objective lens can distinguish two very close object points, it is still impossible to see clearly because the distance between the images of these two object points is smaller than the resolution distance of the eyes. Therefore, the eyepiece and the objective lens are not only related to each other, but also restrict each other.