If a zone-plate has to have a principle focal length of 50 cm corresponding to λ = 6 × 10-...

If a zone-plate has to have a principle focal length of 50 cm corresponding to λ = 6 × 10-5 cm, obtain an expression for the radii of different zones. What would be its principle focal length for λ = 5 × 10-5 cm? [mm, 60 cm]