(a)[2]
State what the principle of superposition means.
(b)
(b(i))[3]
Calculate $D$.
(b(ii))[3]
The slit separation is now reduced steadily from $0.16\,\text{mm}$ to $0.04\,\text{mm}$. The distance between the centres of adjacent dark fringes is $x$. On Fig. 4.3, sketch how $x$ varies with slit separation.