In YDSE how many maxima can be obtained on the screen if wavelength of light used is 200 nm and $d=700nm:$

Here, $\lambda$ and d both are comparable. Hence path difference i.e. $\Delta x=d\sin \theta$

For the maxima, $\Delta x=\pm n\lambda$

$\Rightarrow d\sin \theta =\pm n\lambda \Rightarrow \sin \theta =\pm \frac{2n}{7}$

[where only possible values of $n=0,1,2,3$ ]

Hence, total maxima = 7