(a) How far from grains of red sand must you be to position yourself just at the limit of resolving the grains if your pupil diameter is 1.5 mm, the grains are spherical with radius 50 pm, and the light from the grains has wavelength 650 nm?

(b) If the grains were blue and the light from them had wavelength 400nm, would the answer to (a) be larger or smaller?