If the width of the slit in single slit diffrection experiment is doubled, then the central maximum of diffraction pattern becomes

$Width of central maximum in diffraction pattern due to \sin gle slit =\frac{2\lambda D}{d}$

where $\lambda$is the wavelength, D is the distance between screen and slit and a is the slit width. As the slit width a increases, width of central
maximum becomes sharper or narrower. As same energy is distributed over a smaller area. therefore central maximum becomes brighter.