Geometrical Optics

Wavelength of light used in an optical instrument are $${\lambda _1} = 4000\mathop A\limits^ \circ$$ and $${\lambda _2} = 5000\mathop A\limits^ \circ ,$$ then ratio of their respective resolving powers (corresponding to $${\lambda _1}$$ and $${\lambda _2}$$ ) is :