(a(i))[1]
Explain why a single laser together with two slits is used, rather than two lasers, to create a visible fringe pattern on the screen.
(a(ii).1)[1]
State the phase difference between the waves that meet at $B$.
(a(ii).2)[1]
State the phase difference between the waves that meet at $P$.
(a(iii).1)[2]
State the principle of superposition.
(a(iii).2)[1]
Use the principle of superposition to explain the dark fringe at $P$.
(b)[3]
In Fig. 6.1 the distance from the two slits to the screen is $1.8\,\text{m}$. The distance $CP$ is $2.3\,\text{mm}$ and the distance between the slits is $0.25\,\text{mm}$. Calculate the wavelength of the light provided by the laser.