CBSE Class 12 · Wave OpticsLong Answer

(i) A plane wavefront on a single slit (width $a$) produces secondary wavelets (Huygens). Along the axis they reinforce to give a bright central maximum; for direction $\theta$ the path difference $a\sin \theta$ gives minima at $a\sin \theta =n\lambda$ and weak secondary maxima at $a\sin \theta =(2n+1)\frac{\lambda }{2}$. The intensity shape is a dominant central band (twice as wide) flanked by rapidly decreasing side bands (see figure).
(ii) For a concave mirror take $f$ negative; an object between pole and focus has $|u|<|f|$. From the mirror formula

Since $|u|<|f|$, $u-f$ has the sign that makes $v$ positive, i.e. the image is formed behind the mirror — it is $\text{virtual}$. The magnification is

with $|u|<|f|$ this gives $|m|>1$, so the image is $\text{enlarged}$ (and erect).