What factors are related to the magnification of an optical microscope?
The objective lens uses the incident light to carry out the first imaging of the object being inspected, and obtains a real image of the object enlarged; the function of the eyepiece is to amplify the real image enlarged by the objective lens for the second time, and uses the real image enlarged by the objective lens as an object to further enlarge it into a virtual image. And reflect the object image into the eyes of the observer.
The resolution of a microscope refers to the minimum distance between two object points that can be clearly distinguished by the microscope. According to the diffraction theory, the resolution of a microscope objective is
sigma=0.61lamda/N.sinU ~1 where lamda is the wavelength of the light wave used; N is the refractive index of the space where the object is located, N=1 when the object is in the air; U is the aperture angle, that is, the energy emitted from the object point can enter the objective lens The half-angle of the cone vertex angle of the imaged ray cone; NsinU is called the numerical aperture. When the wavelength λ is constant, the resolution depends on the size of the numerical aperture. The larger the numerical aperture, the finer the structure that can be resolved, that is, the higher the resolution. Numerical aperture is an important performance indicator of a microscopic objective lens. It is usually marked on the objective lens barrel housing together with the magnification. For example, 40×0.65 means that the magnification of the objective lens is 40 times and the numerical aperture is 0.65.
Resolution and magnification are two different but related concepts. When the numerical aperture of the selected objective lens is not large enough, that is, the resolution is not high enough, the microscope cannot distinguish the fine structure of the object. At this time, even if the magnification is increased excessively, the image obtained can only be an image with a large outline but unclear details. . This excessive magnification is called ineffective magnification.
Regarding the use of optical microscopes:
1. Picking up and delivering the microscope: ① Hold the microscope arm with your right hand; ② Hold the microscope holder with your left hand; ③ Place it on your chest.
2. Rotation of the microscope: ① The lens tube faces forward and the mirror arm faces backward; ② Place it on the table in front of the observer's seat, leaning to the left side of the body to facilitate the left eye to observe into the eyepiece; ③ Place it inside the table, away from the edge of the table About 5cm.
3. Aiming the light: ① Turn the coarse focus screw to slowly raise the lens barrel, then turn the converter to align the low-magnification objective lens with the light aperture; ② Use your fingers to turn the shutter (or sheet aperture) to align the maximum aperture. Use the quasi-clear aperture, look into the eyepiece with your left eye, and rotate the reflector toward the light source to make the brightness in the field of view uniform and appropriate.
4. Use of low-magnification objective lenses: ① Turn the coarse focusing screw by hand to slowly lower the lens barrel, while looking at the objective lens from the side, and stop when the distance between the objective lens and the glass slide on the stage is 2 to 3 mm. ② Use your left eye to look into the eyepiece (note that your right eye should be open at the same time), and turn the coarse focus screw to slowly raise the lens barrel until you can see the object clearly. If it is not clear, adjust the fine focus screw until it is clear.
5. Use of high-magnification objective lenses: Before using high-magnification objective lenses, you must first use a low-magnification objective lens to find the object to be observed, adjust it to the center of the field of view, then turn the converter and change to a high-magnification lens. After switching to a high-magnification lens, the brightness in the field of view becomes darker, so generally choose a larger aperture and use the concave surface of the reflector, and then adjust the fine focus screw. The number of objects viewed becomes smaller, but their size increases.
6. Use of reflectors: Reflectors are usually used in conjunction with a shutter (or aperture) to adjust the brightness within the field of view. Reflectors have flat and concave surfaces. When facing the light, if the light in the field of view is too strong, use the flat surface of the reflector. If the light is still too strong, use a smaller aperture at the same time; conversely, if the light in the field of view is weak, use a larger aperture or use a reflector. concave surface.
7. Wiping the lens: ① Use special lens cleaning paper; ② When wiping the lens, fold the lens cleaning paper several times, and then wipe it in one direction. Do not wipe it back and forth or rotate it; ③ If the lens is contaminated by oil, you can Put a few drops of xylene on the lens paper and wipe as above.
8. The magnification object of the microscope: the length and width of the object, not the area, nor the volume.
9. Problems with the focal length of the microscope: the distance between the objective lens and the mounting film, and the use of quasi-focus spirals.
10. The moving direction of the object image when using the microscope: On the contrary, that is, wherever the object image is in the field of view, the film will move in that direction.
11. Judgment of foreign objects when using the microscope: on the eyepiece, objective lens or mounting plate, usually by moving the glass (whether it is on the glass slide) and rotating the converter (whether it is on the objective lens), and what remains is on the eyepiece.
12. Placement of the microscope after the experiment: After using the microscope, remove the glass slides and wipe the mechanical parts clean with white gauze; turn the converter so that the two objective lenses are biased to both sides; turn the coarse focus screw to make the lens The tube is lowered to the lowest point, the reflector is put up, covered with red silk cloth, and then the microscope is locked into the box.
