Angular resolution
Resolving power is the ability of a microscope or telescope to measure the angular separation of images that are close together. Angular resolution describes the resolving power of a telescope. Resolution is the minimum distance between distinguishable objects, in microscopy
Point-like sources separated by an angle smaller than the angular resolution can not be resolved. A single optical telescope has an angular resolution less than one arcsecond, but seeing and other atmospheric effects make attaining this very hard. The highest angular resolutions can be achieved by interferometry.
The angular resolution of a telecope can usually be approximated by R = L/D where L is the wavelength of the observed radiation and D is the diameter of the telescope. The resulting R is in radians. Sources larger than the angular resolution are called extended sources or diffuse sources, and smaller sources are called point sources.
The resolution D depends on the angular aperture α:
Due to the limitations of the values α, λ, and N, the limit of a light microscope using visible light is 200nm. This is because: α for the best lens is 70° (sinα=0.94), shortest wavelength of visible light is blue =450nm and
Telescope case
Microscope case
Here α depends on the width of objective lens and its distance from the specimen; and N is a measure of the number of degrees to which a medium bends a light ray which passes through it. The shorter λ, the lower the value of D, the higher the resolution.