The imaging (geometric imaging) principle of optical microscopy
Only when the angle of the object to the human eye is not less than a certain value, can the naked eye distinguish its various details, and this quantity is called visual resolution ε. Under optimal conditions, where the illumination of an object is 50-70lx and its contrast is high, it can reach 1 '. For ease of observation, this amount is generally increased to 2 'and taken as the average eyepiece resolution.
The size of an object's perspective is related to its length and distance from the object to the eye. There is a formula
y=Lε
The distance L cannot be very small because the eye's ability to adjust has a certain limit, especially when working close to the limit of its ability, it can cause extreme fatigue in vision. For standard (front view), the optimal visual distance is defined as 250mm (clear view distance). This means that without instruments, eyes with a visual resolution of ε=2 'can clearly distinguish details of objects with a size of 0.15mm.
When observing objects with a viewing angle less than 1 ', magnifying instruments must be used. Magnifying glasses and microscopes are used to observe objects that should be magnified when placed close to the observer.
(1) The imaging principle of magnifying glasses
Optical lenses made of glass or other transparent materials with curved surfaces can magnify and image objects. The optical path diagram is shown in Figure 1. Object AB, located within the focal point F of the object, has a size of y and is magnified into a virtual image A'B 'of size y.
The magnification of the magnifying glass
Γ=250/f'
In the formula, 250- visible distance, in millimeters
F '- Magnifier focal length, in mm
The magnification refers to the ratio of the viewing angle of an object image observed with a magnifying glass at a distance of 250mm to the viewing angle of an object not observed with a magnifying glass.
(2) The imaging principle of a microscope
Microscopes and magnifying glasses play the same role, which is to magnify small objects in close proximity for human observation. It's just that a microscope can have a higher magnification than a magnifying glass.
The schematic diagram of an object being imaged by a microscope. For the convenience of calculation, both objective lens L1 and eyepiece lens L2 are represented as single lenses in the figure. Object AB is located in front of the objective lens, and the distance from the objective lens is greater than the focal length of the objective lens, but less than twice the focal length of the objective lens. So, after passing through the objective lens, it will inevitably form an inverted magnified real image A'B '. A'B 'is located at the focal point F2 of the eyepiece or very close to F2. After being magnified by an eyepiece, it becomes a virtual image A''B '' for observation by the eyes. The position of the virtual image A'B 'depends on the distance between F2 and A'B', which can be at an infinite distance (when A'B 'is on F2) or at the observer's bright distance (when A'B' is to the right of the focal point F2 in the figure). The function of an eyepiece is the same as that of a magnifying glass. The only difference is that what the eyes see through the eyepiece is not the object itself, but the magnified image of the object formed by the objective lens.
(3) Important optical technical parameters of microscopes
During microscopy, people always hope to have a clear and bright ideal image, which requires the optical technical parameters of the microscope to meet certain standards, and it is required that the relationship between the parameters must be coordinated according to the purpose and actual situation of microscopy during use. Only in this way can the performance of the microscope be fully utilized and satisfactory microscopy results be achieved.
The optical technical parameters of a microscope include numerical aperture, resolution, magnification, depth of focus, field of view width, coverage difference, working distance, and so on. These parameters are not always better as they are interrelated and mutually restrictive. When using them, the relationship between parameters should be coordinated based on the purpose of microscopy and the actual situation, but the resolution should be guaranteed as the standard.
