What are the main applications of optical microscopes
Optical microscope is an ancient and young scientific tool. Since its birth, it has a history of three hundred years. Optical microscopes are widely used, such as in biology, chemistry, physics, astronomy, etc. In some scientific research work It's all inseparable from the microscope.
At present, it has almost become an image endorsement of science and technology. You only need to see his frequent appearances in media reports about science and technology to see that this is true.
In biology, the laboratory is inseparable from this experimental instrument, which can help learners to study the unknown world; to understand the world.
Hospitals are the largest application places for microscopes. They are mainly used to examine changes in patients' body fluids, bacteria that invade the human body, changes in cell structure, etc., and provide doctors with reference and verification methods for formulating treatment plans. In microsurgery, the microscope is the doctor's only tool; 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 study of material properties are possible. A place to show their talents; criminal investigators often rely on microscopes to analyze various microscopic crimes, as an important means to determine the real culprit; environmental protection departments also use microscopes to detect various solid pollutants; geological and mining engineers and cultural relics and archaeologists use the help of microscopes. The clues found by the microscope can be used to judge the deep underground mines or infer the true image of the dusty history; even people's daily life is inseparable from the microscope, such as the beauty and hairdressing industry, which can use the microscope to detect the skin, hair, etc. Get the 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, and there are four common 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 microscopic bodies; metallographic microscopes are mainly used to observe the surface of opaque objects, such as the metallographic structure and surface defects of materials; When the object is enlarged and imaged, it also makes the orientation of the object and the image relative to the human eye consistent, and has a sense of depth, which is in line with people's conventional visual habits; the polarized light microscope uses the transmission or reflection characteristics of different materials to 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 a biological microscope mainly used to observe the culture through the bottom of the culture vessel; the fluorescence microscope uses certain substances to absorb specific shorter wavelength light and The characteristics of emitting specific longer wavelength light, to find the existence of these substances and determine their content; comparison microscopes can form side-by-side 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 the mechanical structures that support them. The optical systems include objective lenses, eyepieces and condensers, which are complicated magnifying glasses made of various optical glasses. The objective lens magnifies the specimen, and its magnification M 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, forming a virtual image at 250mm in front of people's eyes for observation. This is the most comfortable observation position for most people. The magnification of the eyepiece is 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=Mobject*Meye=Δ*250∕f'eye*f;object. It can be seen that reducing the focal length of the objective lens and eyepiece will increase the total magnification, which is the key to seeing microorganisms such as bacteria with a microscope, and it is also the difference between it and ordinary magnifying glasses.
So, is it conceivable to infinitely reduce the f' object f' mesh in order 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 an electromagnetic wave, so diffraction and interference will inevitably occur during the propagation process, just like the ripples on the water surface that we see in daily life can detour when encountering obstacles, and when two columns of water waves meet, they can strengthen each other. or weakened. When the light wave emitted from a point-shaped light-emitting object point 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 halos with weak and gradually weakening intensity. We call the central bright spot an Airy disk. When the 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 criterion, which is 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 ∕n.sinA=0.61 In ∕ N.A , in the formula, in is the wavelength of the light wave, the wavelength of the light wave that the human eye can receive is about 0.4-0.7um, n is the refractive index of the medium where the light-emitting point is located, such as in the air, n≈1, in the water , n≈1.33, and A is half of the opening angle of the luminous 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 the two points that the objective lens can distinguish is limited by the wavelength of light and the numerical aperture. Since the wavelength of the sharpest human eye is about 0.5um, the angle A cannot exceed 90 degrees, and sinA is always less than 1. The maximum refractive index for the available light-transmitting medium is about 1.5, so the e value is always greater than 0.2um, which is the smallest limit distance that an optical microscope can resolve. Through microscope magnification, if you want to magnify the object point distance e that can be resolved by an objective lens with a certain N.A value enough to be distinguished by the human eye, M.e ≥ 0.15mm, where 0.15mm is the experimentally obtained human eye The minimum distance between two micro-objects placed 250mm in front of the eyes that can be distinguished, 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 M, that is, 500N. A≤M≤1000N.A is a reasonable selection range for 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. Small objects are detailed.
Imaging contrast is another key issue in optical microscopes. The so-called contrast is the black and white contrast or color difference between adjacent parts on the image surface. It is difficult for the human eye to judge the difference in brightness below 0.02. slightly more sensitive. Some microscope observation objects, such as biological specimens, have very little difference in brightness between details. In addition, the design and manufacturing errors of the microscope optical system further reduce the imaging contrast and make it difficult to distinguish. At this time, the details of the object cannot be seen clearly, not because the total magnification is too low. , it is not because the numerical aperture of the objective lens is too small, but because the image surface contrast is too low.
Over the years, people have worked hard to improve the resolving power and imaging contrast of microscopes. With the continuous advancement of computer technology and tools, the theory and methods of optical design are also constantly improving. The continuous improvement of detection methods and the innovation of observation methods have made the imaging quality of optical microscopes close to the perfect degree of diffraction limit. It can adapt to the research of all kinds of specimens. Although the magnifying and imaging instruments such as electron microscope and ultrasonic microscope have come out successively in recent years, they have advantageous performance in some aspects, but they still cannot be cheap, convenient and intuitive, especially suitable for the research of living organisms. Rivaling light microscopes, which still hold their ground firmly. On the other hand, combined with laser, computer, new material technology and information technology, the ancient optical microscope is rejuvenating and showing strong vitality. Digital microscope, laser confocal scanning microscope, near-field scanning microscope, two-photon microscope and Instruments with various new functions or adaptable to various new environmental conditions emerge in an endless stream, further expanding the application field of optical microscopes, as an example. How exciting are the microscopic pictures of rock formations uploaded from the Mars rover! We can fully believe that the optical microscope will benefit mankind with a new attitude.
