What are the factors related to the magnification of an optical microscope?
The objective lens uses the incident light to pass through the object to be inspected for the first imaging, and obtains a real image enlarged by the object; the function of the eyepiece is to amplify the real image enlarged by the objective lens for the second time. And reflect the image into the eyes of the observer.
The resolution of a microscope is the smallest distance between two object points that can be clearly distinguished by the microscope. According to 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, and N=1 when the object is in the air; U is the aperture angle, that is, the emission from the object point can enter the objective lens The half angle of the apex 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 index of the microscope objective, which is usually marked on the objective lens barrel 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 distinct 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, only an image with a large outline but unclear details can be obtained. . This excessive magnification is called ineffective magnification.
Regarding the use of optical microscopes:
1. Pick-up and delivery of the microscope: ① Hold the mirror arm with the right hand; ② Hold the mirror holder with the left hand; ③ Place it on the chest.
2. Rotation of the microscope: ① the lens barrel is facing forward and the mirror arm is facing back; ② it is placed on the table in front of the observer's seat, and it is inclined to the left side of the body to facilitate the observation of the left eye into the eyepiece; ③ it is placed on the inside of the table, away from the edge of the table About 5cm.
3. Alignment: ①Turn the coarse focusing screw to make the lens barrel rise slowly, and then turn the converter to make the low magnification objective lens align with the light aperture; ②Use your finger to turn the shutter (or flake aperture) to make the maximum aperture correct. With the quasi-light hole, the left eye looks into the eyepiece, and at the same time, the reflector is turned to face the light source, so that the brightness in the field of view is even and appropriate.
4. Use of low magnification objective lens: ①Turn the coarse focus screw by hand to make the lens barrel descend slowly, while watching the objective lens from the side with both eyes, stop when the distance between the objective lens and the glass on the stage is 2-3mm. ②Look into the eyepiece with your left eye (note that your right eye should be open at the same time), and turn the coarse focus screw to make the lens barrel rise slowly 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 lens: Before using high magnification objective lens, you must first find the observed object with low magnification objective lens, adjust it to the center of the field of view, and then turn the converter to change the high magnification lens. After switching to a high-power lens, the brightness in the field of view becomes darker, so a larger aperture is generally used and the concave surface of the reflector is used, and then the fine focus screw is adjusted. The number of objects viewed becomes smaller, but the volume becomes larger.
6. Use of Mirrors: Mirrors are usually used in conjunction with a shutter (or aperture) to adjust the brightness within the field of view. Mirrors have flat and concave surfaces. When looking at the light, if the light in the field of view is too strong, use the plane of the mirror. If the light is still too strong, use a smaller aperture at the same time; on the contrary, if the light in the field of view is weak, use a larger aperture or use a mirror the concave surface.
7. Lens cleaning: ①Use special lens cleaning paper; ②When wiping the lens, first fold the lens cleaning paper several times, then wipe it in one direction, do not wipe it back and forth or rotate it; ③If the lens is contaminated with oil, you can Put a few drops of xylene on lens tissue and wipe as above.
8. The magnifying object of the microscope: it is the length and width of the object, not the area, nor the volume.
9. The focal length of the microscope: the distance between the objective lens and the mount, the use of the focal spiral.
10. The moving direction of the object image when the microscope is in use: On the contrary, that is, where the object image is in the field of view, the film will move in this direction.
11. Judgment of foreign objects when the microscope is in use: On the eyepiece, objective lens or mount, it is usually judged by moving the glass (whether it is on the glass) and turning the converter (whether it is on the objective), and the rest is on the eyepiece.
12. The placement of the microscope after the experiment: after the microscope is used, the glass slide should be taken off, and its mechanical part should be wiped clean with white gauze; The tube is lowered to the lowest point, the mirror is erected, covered with a red silk cloth, and then the microscope is locked in the box.
