Why is it that the shorter the eyepiece in the microscope, the larger the magnification, the longer the objective?
answer:
Microscope principle: The object AB becomes a real image A'B' through the objective lens, A 'B' is located at F2 in front of the eyepiece (or within the front focal surface of the eyepiece, near the focal surface); A 'B' passes through the eyepiece to emit nearly parallel light, generating a positive magnification of the virtual image.
In the design of the microscope, in order to achieve the convenience and speed of operation (that is, in the case of keeping the object position still, the magnification of the objective and the eyepiece can always be replaced, and it is always possible to obtain a clear image, and it is located in the middle of the field of view), it is necessary to ensure that the image surface A'B' of the objective is always coincident with the front focal surface F2 of the eyepiece, and there is no need to refocus, and the relative position of the objective and the eyepiece meets the conditions of homo focus.
Homofocal conditions: Optical barrel length Δ varies with the focal length f1' of the objective lens, but the conjugate distance M from the object plane to the image plane is determined and unchanged. When selecting different optical tube lengths Δ, the M value should be kept unchanged.
Simply put, because the higher the lens magnification, the shorter the focal length, the shorter the working distance of the high-magnification objective imaging W.D., the closer to the sample, so the lens is longer; in order to maintain the eyepiece focal length coincides with the object mirror surface A'B', the high-magnification eyepiece lens is shorter.
Resources:
The National University of Science and Technology applied optical PPT
CHI Zeying, CHEN Wenjian. Fundamentals of Applied Optics and Optical Design[M]. Higher Education Press, 2013.
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