The main parameters of the objective lens of an optical microscope
The main parameters of the objective lens are:
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 length rather than the ratio of area. Example: Magnification factor of 100 ×， Refers to a length of 1 μ The length of the magnified specimen of m is 100 μ m. If calculated by area, it is magnified by 10000 times.
The total magnification of a microscope is equal to the product of the magnification of the objective and eyepiece.
② Numerical aperture, also known as aperture ratio, abbreviated as NA or A, is the main parameter of the objective lens and condenser, and is directly proportional to the resolution of the microscope. The numerical aperture of the dry objective is 0.05-0.95, and the numerical aperture of the oil immersed objective (cedar oil) is 1.25.
optical microscope
③ Working distance refers to the distance from the front lens of the objective lens to the cover glass of the specimen when the specimen being observed is the clearest. The working distance of the objective lens is related to its focal length. The longer the focal length of the objective lens, the lower the magnification, and the longer its working distance. Example: A 10x objective is labeled with 10/0.25 and 160/0.17, where 10 is the magnification of the objective; 0.25 is the numerical aperture; 160 is the length of the lens tube (in mm); 0.17 is the standard thickness of the cover glass in millimeters. The effective working distance of a 10x objective is 6.5mm, and the effective working distance of a 40x objective is 0.48mm.
Why use an oil mirror to achieve the highest resolution in optical microscopy
Oil mirror, one of the optical microscopes, is used when the lens is immersed in oil (usually cedar oil) to observe finer structures. It is one of the commonly used microscopes in the laboratory, with slightly higher clarity than ordinary optical microscopes, and is used to observe chlamydia, bacteria, organelles, etc
The lens of the oil lens is very small. When light passes through the air between the glass slide and the oil lens, refraction or total reflection occurs due to different media densities, reducing the amount of light entering the lens and blurring the image. If cedar oil (n=1.515) with a refractive index similar to that of the glass (n=1.52) is added between the oil lens and the glass slide, the amount of light entering the lens increases, the brightness of the field of view increases, and the image becomes bright and clear
Due to the small size of bacteria, in the study of bacterial morphology, it is often necessary to use a microscope oil mirror to observe them clearly. Therefore, it is necessary to proficiently master the use and protection methods of oil mirrors
