Which sectors make the most use of optical microscopes?
Hospitals are the largest application places for microscopes, which are mainly used to check information such as changes in patient's body fluids, germs invading the human body, changes in cell tissue structure, etc., and provide doctors with reference and verification methods for formulating treatment plans. In surgery, the microscope is the most important tool for doctors; in agriculture, breeding, pest control and other work cannot do without the help of the microscope; in industrial production, the processing inspection and assembly adjustment of fine parts, and the research of material properties Skills; Criminal investigators often rely on microscopes to analyze various microscopic crimes, as an important means to determine the real murderer; Environmental protection departments also need microscopes when detecting various solid pollutants; Geological and mining engineers and cultural relics and archaeologists use microscopes The clues found can judge the deep underground mineral deposits or infer the dusty historical truth; even people's daily life cannot do without microscopes, such as the beauty and hairdressing industry, which can use microscopes to detect skin and hair quality. For best results. It can be seen how closely the microscope is integrated with people's production and life.
According to different application purposes, microscopes can be roughly classified into four categories: biological microscopes, metallographic microscopes, stereo microscopes, and polarizing microscopes. As the name implies, biological microscopes are mainly used in biomedicine, and the observation objects are mostly transparent or translucent micro bodies; metallographic microscopes are mainly used to observe the surface of opaque objects, such as the metallographic structure and surface defects of materials; While the object is magnified and imaged, the orientation of the object and the image relative to the human eye is also consistent, and there is a sense of depth, which is in line with people's conventional visual habits; Polarizing microscopes use the transmission or reflection characteristics of different materials for polarized light to distinguish different micro objects Component. In addition, some special types can also be subdivided, such as an inverted biological microscope or a culture microscope, which is mainly used to observe the culture through the bottom of the culture vessel; a fluorescence microscope uses certain substances to absorb specific shorter wavelength light The characteristics of emitting specific longer wavelength light to discover the existence of these substances and judge their content; the comparison microscope can form juxtaposed or superimposed images of two objects in the same field of view, so as to compare the similarities and differences of the two objects.
Traditional optical microscopes are mainly composed of optical systems and their supporting mechanical structures. The optical systems include objective lenses, eyepieces and condenser lenses, all of which are complicated magnifying glasses made of various optical glasses. The objective lens enlarges the image of the specimen, and its magnification M object is determined by the following formula: M object = Δ∕f' object , where f' object is the focal length of the objective lens, and Δ can be understood as the distance between the objective lens and the eyepiece. The eyepiece magnifies the image formed by the objective lens again, and forms a virtual image at 250mm in front of the human eye for observation. This is the most comfortable observation position for most people. The magnification of the eyepiece M eye=250/f' eye, f' eye is the eyepiece focal length. The total magnification of the microscope is the product of the objective lens and the eyepiece, that is, M=M object*M eye=Δ*250/f' eye *f; object. It can be seen that reducing the focal length of the objective lens and the eyepiece will increase the total magnification, which is the key to seeing bacteria and other microorganisms with a microscope, and it is also the difference between it and ordinary magnifying glasses.
So, is it conceivable to reduce the f' object f' mesh without limit, so as to increase the magnification, so that we can see more subtle objects? The answer is no! This is because the light used for imaging is essentially a kind of electromagnetic wave, so diffraction and interference phenomena will inevitably occur during the propagation process, just like the ripples on the water surface that can be seen in daily life can go around when encountering obstacles, and two columns of water waves can strengthen each other when they meet Or weaken the same. When the light wave emitted from a point-shaped luminous object enters the objective lens, the frame of the objective lens hinders the propagation of light, resulting in diffraction and interference. There is a series of light rings with weak and gradually weakening intensity. We call the central bright spot as the Airy disk. When two light-emitting points are close to a certain distance, the two light spots will overlap until they cannot be confirmed as two light spots. Rayleigh proposed a judgment standard, thinking that when the distance between the centers of the two light spots is equal to the radius of the Airy disk, the two light spots can be distinguished. After calculation, the distance between the two light-emitting points at this time is e=0.61 入/n.sinA=0.61 I/N.A, where I is the wavelength of light, the wavelength of light that can be received by the human eye is about 0.4-0.7um, and n is the refractive index of the medium where the light-emitting point is located, such as in air, n≈1, in water , n≈1.33, and A is half of the opening angle of the light-emitting point to the frame of the objective lens, and N.A is called the numerical aperture of the objective lens. It can be seen from the above formula that the distance between two points that can be distinguished by the objective lens is limited by the wavelength of light and the numerical aperture. Since the wavelength of the most acute vision of the human eye is about 0.5um, and the angle A cannot exceed 90 degrees, sinA is always less than 1. The maximum refractive index of the available light-transmitting medium is about 1.5, so the e value is always greater than 0.2um, which is the minimum limit distance that the optical microscope can distinguish. Magnify the image through a microscope, if you want to magnify the object point distance e that can be resolved by the objective lens with a certain N.A value enough to be resolved by the human eye, you need M.e ≥ 0.15mm, where 0.15mm is the experimental value of the human eye The minimum distance between two micro-objects that can be distinguished at 250mm in front of the eyes, so M≥ (0.15∕0.61 in) N.A≈500N.A, in order to make the observation not too laborious, it is enough to double the M, that is, 500N. A≤M≤1000N.A is a reasonable selection range of the total magnification of the microscope. No matter how large the total magnification is, it is meaningless, because the numerical aperture of the objective lens has limited the minimum resolvable distance, and it is impossible to distinguish more by increasing the magnification. Small objects are detailed.
