Optical microscopes are quoted as having a maximum magnification of 1500x to 2000x - what is this calculated from?
If you stick 200x lens and 20x eyepiece - you theoretically can have 4000x magnification, but you would not be able to see more details compared to 100x lens and 20x eyepiece, because resolving the smallest visible details is limited to Rayleigh criterion (i.e. limited to diffraction).
Where λ is wavelength in nm, and NA is numerical aperture of the lens. So for violet light λ=405nm, and good lens with oil immersion (NA=1.25), you can have resolution 197nm.
So, in conclusion, optical microscopes are limited to ~x1500 because going any further does not resolve smaller details.
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