Question

the central bright maxima is twice as wide as the other maxima.

Answer 1

Consider a monochromatic beam of light incident on a slit of width a. Interference occurs between the light that passes through different parts of the slit, resulting in what is called a diffraction pattern. Viewed on a distant screen, the diffraction pattern consists of alternating bright and dark interference fringes. The central bright fringe is twice as wide, and much brighter, than the other bright fringes. Central bright fringe can be explained by wave theory of light, using the principle of constructive interference.
According to Huygen's principle, each portion of the slit acts as a source of light waves. Hence, light from one portion of slit can interfere with lights of other portion, and the resultant intensity depends on the direction, $\theta$. Centres of first dark fringes will be found at angle of $sin{\theta }_{dark}=\pm \frac{\lambda }{a}$
Between these two, the central maxima is found with a width of $\frac{2\lambda }{a}$. Others will lie between $\frac{n\lambda }{a}$ and $\frac{(n+1)\lambda }{a}$, thus having a width of only $\frac{\lambda }{a}$.
Hence central maxima is twice in width.