How to see the magnification of the eyepiece and objective lens of an optical microscope
The magnification of an optical microscope is the product of the magnification of the objective lens and the magnification of the eyepiece. For example, if the objective lens is 10× and the eyepiece is 10×, the magnification is 10×10=100.
One objective:
1. Classification of objective lenses:
The objective lens can be divided into dry objective lens and liquid immersion objective lens according to different conditions of use; among which liquid immersion objective lens can be divided into water immersion objective lens and oil immersion objective lens (commonly used magnification is 90-100 times).
According to the different magnification, it can be divided into low magnification objective lens (below 10 times), medium magnification objective lens (about 20 times) and high magnification objective lens (40-65 times).
According to the aberration correction situation, it is divided into achromatic objective lens (commonly used objective lens that can correct the chromatic aberration of two colors in the spectrum) and achromatic objective lens (objective lens that can correct the chromatic aberration of three color lights in the spectrum, expensive and less used).
2. The main parameters of the objective lens:
The main parameters of the objective lens include: magnification, numerical aperture and working distance.
①, magnification refers to the ratio of the size of the image seen by the eye to the size of the corresponding specimen. It refers to the ratio of lengths rather than the ratio of areas. Example: The magnification is 100×, which means that the length of the specimen is 1 μm. The length of the enlarged image is 100 μm. If it is calculated by area, it is magnified 10,000 times.
The total magnification of a microscope is equal to the product of the objective and eyepiece magnifications.
②. Numerical aperture is also called lens ratio, abbreviated as NA or A, which is the main parameter of objective lens and condenser, and is proportional to the resolving power of microscope. Dry objectives have a numerical aperture of 0.05-0.95, and oil immersion objectives (cedar oil) have a numerical aperture of 1.25.
③. Working distance refers to the distance from the bottom of the front lens of the objective lens to the top of the cover glass of the specimen when the observed specimen is the clearest. The working distance of the objective lens is related to the focal length of the objective lens. The longer the focal length of the objective lens, the lower the magnification, and the longer the working distance. Example: 10x objective lens is marked with 10/0.25 and 160/0.17, of which 10 is the magnification of the objective lens; 0.25 is the numerical aperture; 160 is the length of the lens barrel (unit mm); 0.17 is the standard thickness of the cover glass (unit mm) ). The effective working distance of the 10x objective is 6.5mm, and the effective working distance of the 40x objective is 0.48mm.
3. The function of the objective lens is to magnify the specimen for the first time. It is the most important component that determines the performance of the microscope - the level of resolution.
Resolution is also called resolution or resolution. The size of the resolution is expressed by the value of the resolution distance (the minimum distance between two object points that can be resolved). At the photopic distance (25cm), the normal human eye can clearly see two object points that are 0.073mm apart. The value of 0.073mm is the resolution distance of the normal human eye. The smaller the resolution distance of the microscope, the higher its resolution, which means the better its performance.
The resolution of the microscope is determined by the resolution of the objective lens, and the resolution of the objective lens is determined by its numerical aperture and the wavelength of the illumination light.
When using the ordinary central illumination method (photopic illumination method that makes the light pass through the specimen uniformly), the resolution distance of the microscope is d=0.61λ/NA
where d is the resolution distance of the objective lens, in nm.
λ——the wavelength of illumination light, in nm.
NA - the numerical aperture of the objective lens
For example, the numerical aperture of the oil immersion objective is 1.25, the visible light wavelength range is 400-700nm, and the average wavelength is 550 nm, then d=270 nm, which is about half the wavelength of the illumination light. Generally, the limit of microscope resolution with visible light illumination is 0.2 μm.
(2) Eyepieces
Because it is close to the observer's eye, it is also called an eyepiece. Mounted on the upper end of the lens barrel.
1. eyepiece structure
Usually the eyepiece is composed of two sets of upper and lower lenses, the upper lens is called the eye lens, and the lower lens is called the converging lens or field lens. A diaphragm is installed between the upper and lower lenses or under the field lens (its size determines the size of the field of view), because the specimen is just imaged on the diaphragm surface, a small piece of hair can be glued on this diaphragm as a pointer to indicate target of a certain characteristic. An eyepiece micrometer can also be placed on it to measure the size of the specimen being observed.
The shorter the length of the eyepiece, the greater the magnification (because the magnification of the eyepiece is inversely proportional to the focal length of the eyepiece).
2. The role of eyepieces
It is to further magnify the real image that has been magnified by the objective lens and can be clearly distinguished to the extent that the human eye can easily distinguish it. Commonly used eyepiece magnification is 5-16 times.
3. The relationship between eyepiece and objective lens
The fine structure that the objective lens has already resolved clearly, if it is not re-magnified by the eyepiece and cannot reach the size that the human eye can distinguish, then it will not be able to see clearly; but the fine structure that cannot be distinguished by the objective lens, although it is re-magnified by the high-power eyepiece, It is still not clear, so the eyepiece can only magnify, and will not improve the resolution of the microscope. Sometimes, although the objective lens can distinguish two object points that are close to each other, it is still impossible to see clearly because the distance between the images of these two object points is smaller than the resolving distance of the eye. Therefore, the eyepiece and the objective lens are interrelated and restrict each other.
