In a Young's double slit experiment, first maxima is observed at a fixed point P on the screen. Now the screen is continuously moved away from the plane of slits. The ratio of intensity at point P to the intensity at point O (centre of the screen)

${\mathrm{I}}_{\mathrm{p}}=\frac{{\mathrm{I}}_{max}}{2}[1+\mathrm{cos\varphi }]=\frac{{\mathrm{I}}_{max}}{2}\left(1+\cos \frac{2\mathrm{\pi y}}{\mathrm{\beta }}\right)$

Where $\mathrm{\beta }=\frac{\mathrm{D\lambda }}{\mathrm{d}}$

First maxima is observed at P i.e., $\cos \frac{2\mathrm{\pi y}}{\mathrm{\beta }}=1;\text{ as }\mathrm{D}$ increases D will increase, the value of $\cos \frac{2\mathrm{\pi y}}{\mathrm{\beta }}$ should be negative hence the ratio ${\mathrm{I}}_{\mathrm{P}}/{\mathrm{I}}_{max}$ starts decreasing but starts increasing again as $\cos \frac{2\mathrm{\pi y}}{\mathrm{\beta }}$ again starts becoming positive.