Only when the opening angle of the object to the human eye is not less than a certain value, the naked eye can distinguish its various details, which is called the visual resolution ε. Under the best conditions, that is, when the illumination of the object is 50~70lx and its contrast is large, it can reach 1'. For easy observation, this amount is generally increased to 2', and this is taken as the average eyepiece resolution.
The size of the viewing angle of an object is related to the length of the object and the distance from the object to the eye. There is the formula y=Lε
The distance L cannot be made very small, because the accommodation ability of the eyes has a certain limit, especially when the eyes work close to the limit range of the accommodation ability, the eyesight will be extremely fatigued. For the standard (front view), the optimal viewing distance is specified as 250mm (photopic distance). This means that under the condition of no instrument, the eye with visual resolution 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 placed near the observer that should be magnified.
(1) Imaging principle of magnifying glass
An optical lens made of glass or other transparent materials with a curved surface can magnify and image objects. The optical path diagram is shown in Figure 1. The object AB located within the focal point F of the object side, and its size is y, is transformed into a virtual image A'B' of size y' by the magnifying glass.
magnification of magnifying glass
Γ=250/f'
In the formula, 250--photopic distance, the unit is mm
f'-- the focal length of the magnifying glass, in mm
The magnification refers to the ratio of the viewing angle of the object image observed with a magnifying glass to the viewing angle of the object observed without a magnifying glass within a distance of 250mm.
(2) The imaging principle of the microscope
A microscope and a magnifying glass have the same function, that is, to turn a small object in the vicinity into a magnified image for the human eye to observe. It's just that a microscope can have a higher magnification than a magnifying glass.
Figure 2 is a schematic diagram of an object being imaged by a microscope. In the figure, for convenience, both the objective lens L1 and the eyepiece L2 are represented by a single lens. Object AB is located in front of the objective lens at a distance 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 focal point F2 of the eyepiece, or very close to F2. Then magnify it into a virtual image A''B'' through the eyepiece for eye observation. 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 on F2) or at the observer's photopic distance ( When A'B' is to the right of focus F2 in the figure). The eyepiece acts like a magnifying glass. The difference is that what the eye sees through the eyepiece is not the object itself, but the image of the object that has been enlarged once by the objective lens.
