Monochromatic light waves of wavelength $\lambda$ from two coherent sources fall on a large plane screen. One of the waves is emanated from a point source S located at distance L from the screen and other one is a broad plane wave as shown in the figure. 

Mark the correct options 

$$\begin{gathered} Let say n^{th} bright fringe is forming at point P \\ Path difference, \Delta x = SP- RP = n\lambda \\ \Rightarrow \Delta x=\sqrt{L^2+r^2}- L=n\lambda \\ \Rightarrow \sqrt{L^2+r^2}=n\lambda +L \\ \Rightarrow L^2+r^2=n^2{\lambda }^2+L^2+2n\lambda L \\ \Rightarrow r=\sqrt{2n\lambda L } [\because n^2{\lambda }^2 may be neglected] \\ \therefore r_n = \sqrt{2n\lambda L } and r_{n-1}=\sqrt{2(n-1)\lambda L} \\ Spacing between them = r_n - r_{n-1} \\ = \sqrt{2n\lambda L }- \sqrt{2(n-1)\lambda L} \\ =\sqrt{2\lambda L}(\sqrt{n}-\sqrt{n-1}) \end{gathered}$$