The shape of wavefront of light diverging from point source

The limit of resolution of a telescope is $3.0 \times 10^{-7} \mathrm{rad}$. Assuming that it is used to see the light of wavelength 525 nm from a star, what should be the diameter of the objective?

On using red light $(\lambda=6600 \mathop {\rm{A}}\limits^{\rm{o}})$ in Young's double slit experiment, 60 fringes are observed in the field of view. If violet light ( $\lambda=4400 \mathop {\rm{A}}\limits^{\rm{o}}$ ) is used, the number of fringes observed will be

Young's double slit experiment is carried out by using green, red and blue light, one colour at a time. The fringe width recorded are $\beta_{G^{\prime}} \beta_{R^{\prime}} \beta_B$ respectively, then