Question

What is limit of resolution for microscopes? I want to know the formulae

Answer 1

The resolution of the lightmicroscope cannot be small than the half of the wavelength of the visible light, which is 0.4-0.7 µm. When we can see green light (0.5 µm), the objects which are, at most, about 0.2 µm. Below this point, light microscope is not useful, as wavelength smaller than 400 nm is needed.
FORMULA
A microscope's resolution limit, d, can be found by the following formula: d = 0.61 λ / NA