resolving power All optical instruments give images that are affected by diffraction at the objective lens. The images of two close objects overlap and sometimes they cannot be seen as separate.

Rayleigh criterion: The two images are just resolved when central maximum of one diffraction pattern is not closer than the first minimum of the second diffraction pattern.

For diffraction due to rectangular aperture,

l = d sinq

where q is the angle of first minimum, and d the width of the slit. As q is small,

q = l /d

For circular aperture, the condition is,

q = 1.22 l /d (r17)

where d is the diameter. Higher resolution is possible with large aperture or short wavelength of light.