$($b$)$ Coherent light is diffracted by a single slit which is $5m$ wide. The central maximum of the diffracted light has a width of $1\mathrm{.}5$ cm when it falis on a screen $0\mathrm{.}3$ m behind the slit. Determine the wavelength of the incident light.

$(3$ marks$)$

$($c$)$ Two slits on a screen placed $0\mathrm{.}1$ mm apart are $0\mathrm{.}8$ m from a second screen. The slits are illuminated by coherent light of wavelength $600$ nm $.$ What is the distance between the $m=2$ fringes above and below the central maximum of this interference pattern?

$(3$ marks$)$

$($d$)$ If the minimum intensity of the signal to be received at a distance of $120$ m from a domestic radio transmitter is $8W{m}^{-2},$ calculate the required transmitter power.

$(2$ marks$)$