Angular resolution in radians = 1.22 times the quotient (wavelength / aperture)
Make sure to keep the units the same, wavelength may be in angstroms; aperture may in inches. So, do the conversions first.
For example, a telescope with a 10-inch lens is assumed to be observing in the visible spectrum. Assume the middle range. The last step is to convert radians to arc seconds.
Answer: 0.54 arc seconds.
My telescope has an aperture of 102 mm.
Answer ranges 0.98″ (violet) to 1.72″ (red)