The working principle of optical microscope: Microscope is a precision optical instrument with a development history of more than 300 years. Since the advent of microscopes, people have been able to see many tiny organisms and the basic building blocks of life - cells - that were previously unseen. At present, there are not only optical microscopes that can magnify over a thousand times, but also electron microscopes that can magnify hundreds of thousands of times, which have given us a further understanding of the laws of biological activity. In the biology teaching syllabus of ordinary high schools, most experiments are required to be completed through a microscope. Therefore, the performance of the microscope is the key to conducting observation experiments well.
1、 Optical system of microscope
The optical system of a microscope mainly includes four components: objective lens, eyepiece, reflector, and condenser. Broadly speaking, it also includes lighting sources, filters, coverslips, and glass slides.
（1） Objective lens
The objective lens is the most important component that determines the performance of a microscope. It is installed on the objective lens converter and is close to the observed object, so it is called an objective lens or a connecting lens.
1. Classification of objective lenses
Objective lenses can be divided into dry objectives and immersion objectives according to different usage conditions; Among them, immersion lenses can be divided into water immersion lenses and oil immersion lenses (commonly used magnification is 90-100 times).
According to different magnification factors, it can be divided into low magnification objectives (below 10x), medium magnification objectives (around 20x), and high magnification objectives (40-65x).
According to the aberration correction situation, it is divided into achromatic objective lenses (commonly used, which can correct the color difference of two colors in the spectrum) and chromatic aberration objective lenses (which can correct the color difference of three colors in the spectrum, expensive, and rarely used).
2. Main parameters of objective lens:
The main parameters of the objective lens include magnification, numerical aperture, and working distance.
① Magnification refers to the ratio of the size of the image seen by the eyes to the corresponding specimen size. It refers to the ratio of length rather than the ratio of area. Example: A magnification of 100 times refers to a specimen with a length of 1 μ m. The length of the magnified image is 100 μ m. If calculated by area, it is magnified 10000 times.
The total magnification of a microscope is equal to the product of the magnification of the objective lens and eyepiece lens.