Calculate the distance of the first dark ring from the centre of the diffraction pattern.?

A collimated beam of light from an He-Ne laser (λ=632.8 nm) falls normally onto a circular aperture of 0.5 mm diameter. A lens of 0.5 m focal length placed just behind the aperture focuses the diffracted light onto a screen. Calculate the distance of the first dark ring from the centre of the diffraction pattern. Using the same arrangement, what is the minimum separation of another source which could just be resolved if the two sources are 10 m from the aperture?