The basic structure of a microscope and how an oil lens works
Modern general optical microscopes utilize a system of two lenses, an eyepiece and an objective lens, to magnify the image, and are often referred to as compound microscopes. They are composed of two major parts: the mechanical device and the optical system. In the optical system of the microscope, the performance of the objective lens is the most critical, which directly affects the resolution of the microscope. Among the several types of objective lenses commonly used in general optical microscopes, the oil lens has the greatest magnification and is the most important for microbiological research. Compared with other objective lenses, the use of oil lenses is more special, you need to add a drop of mirror oil between the slide and the lens, which is mainly due to the following two aspects:
1. increase the brightness of the illumination of the oil mirror magnification of up to 100 Χ, magnification of such a large lens, the focal length is very short, the diameter is very small, but the intensity of light required is the largest. From carrying specimens of the slide through the light, due to different media density (from the slide into the air, and then into the lens), some of the light will be due to refraction or total reflection, can not enter the lens, resulting in the use of the oil mirror will be due to less light, the object image does not appear clear. So in order not to make the light through the loss, in the use of oil mirror must be added between the oil mirror and the slide with the refractive index of the glass (n = 1.55) similar to the oil mirror (usually with cedar oil, its refractive index n = 1.52).
2. Increase the resolution of the microscope The resolution or resolving power of a microscope is the ability of the microscope to recognize the minimum distance between two points. From a physical point of view, the resolution of an optical microscope is limited by the interference phenomenon and the performance of the objective lens used. The resolving power D can be expressed as follows: D = λ/2N.A, where λ = wavelength of the light wave; NA = numerical aperture value of the objective lens. Optical microscope light source is not possible beyond the wavelength range of visible light (0.4 - 0.7 μ m), while the numerical aperture value depends on the objective lens of the mirror mouth angle and the refractive index of the medium between the slide and the lens, which can be expressed as follows: NA = n × sin α in which α is the half of the maximum angle of incidence of light. It depends on the diameter and focal length of the objective lens, in general, in practice, the maximum can only reach 120 O, and n for the refractive index of the medium. Because the refractive index of cedar oil (1.52) than the refractive index of air and water (respectively, 1.0 and 1.33) to be higher, so the cedar oil as a lens in the slide between the medium of the oil mirror can achieve the numerical aperture value (NA is generally in the 1.2-1.4) to be higher than the low-magnification lens, high-magnification lens, such as dry mirror (NA are lower than 1.0). If the average wavelength of visible light is 0.55 μm, a high magnification lens with a numerical aperture of 0.65 can only distinguish objects at a distance of not less than 0.4 μm, whereas the resolution of an oil mirror can be about 0.2 μm.
