Resolving power of a telescope can be increased by increasing

In Young's double slit experiment, the intensity on screen at a point, where path difference is $\frac{\lambda}{4}$ is $\frac{K}{4}$. The intensity at a point when path difference is ' $\lambda$ ' will be $\left[\cos \frac{\pi}{2}=0, \cos 2 \pi=1\right]$

Graph shows the variation of fringe width ( X ) versus distance of the screen from the plane of the slits (D) in Young's double slit experiment. (keeping other parameters same, $d=$ distance between the slits). The wavelength of light used can be calculated as

In Young's double slit experiment, in an interference pattern second minimum is observed exactly in front of one slit. The distance between the two coherent sources is ' $d$ ' and the distance between source and screen is ' $D$ '. The wavelength of light source used is