Principles of imaging (geometric imaging) of optical microscopes
Only when the opening angle of an object to the human eye is not less than a certain value, the naked eye can distinguish its details. This quantity is called visual resolution ε. Under optimal conditions, that is, when the illumination of the object is 50~70lx and the contrast is large, it can reach 1'. For easier observation, this amount is generally increased to 2', and this is taken as the average eyepiece resolution.
The size of the object's viewing angle is related to the length of the object and the distance from the object to the eye. There is a formula y=Lε
The distance L cannot be made very small because the adjustment ability of the eyes has a certain limit. Especially when the eyes work close to the limit range of the adjustment ability, the vision will be extremely fatigued. For standard (face view), the optimal viewing distance is 250mm (clear vision distance). This means that without instruments, eyes with a visual resolution of ε=2' can clearly distinguish the details of objects with a size of 0.15mm.
When observing objects with a viewing angle less than 1', a magnifying instrument must be used. Magnifying glasses and microscopes are used to observe objects that are placed close to the observer and should be magnified.
(1) The imaging principle of magnifying glass
An optical lens made of glass or other transparent materials with a curved surface can magnify objects into images. The optical path diagram is shown in Figure 1. The object AB located within the object-side focus F has a size of y, and is magnified into a virtual image A'B' of a size of y'.
Magnification of magnifying glass
Γ=250/f'
Where 250--clear vision distance, unit is mm
f'--focal length of magnifying glass, unit is mm
The magnification ratio refers to the ratio of the angle of view of an object image observed with a magnifying glass at a distance of 250 mm to the angle of view of an object observed without a magnifying glass.
(2) Imaging principle of microscope
Microscopes and magnifying glasses play the same role, which is to enlarge nearby small objects into a magnified image for human eyes to observe. It's just that a microscope can have higher magnification than a magnifying glass.
Schematic diagram of an object being imaged by a microscope. For convenience, the figure shows the objective lens L1 and the eyepiece L2 as a single lens. Object AB is located in front of the objective lens, and its 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. Therefore, after it passes through the objective lens, it will inevitably form an inverted magnified real image A'B'. A'B' is located at the object focus F2 of the eyepiece, or very close to F2. It is then magnified into a virtual image A''B'' through the eyepiece 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 infinity (when A'B' is located on F2) or at the observer's apparent distance ( When A'B' is to the right of focus F2 in the figure). Eyepieces function like a magnifying glass. The only difference is that what the eye sees through the eyepiece is not the object itself, but the image of the object that has been magnified once by the objective lens.
