State what diffraction of a wave means.
A beam of vertically polarised light with wavelength $540\,\text{nm}$ is normally incident on a diffraction grating, as illustrated in Fig. 4.1. The diffraction grating has a line spacing of $5.0 \times 10^{-6}\,\text{m}$. Light emerging from the diffraction grating falls on a circular screen. The diffraction grating is located at the centre $X$ of the circle. The central bright fringe appears at point $O$ on the screen and has intensity $I_0$. $P$ is a point on the screen for which the line $XP$ makes a variable angle $\theta$ with the line $XO$. The intensity $I$ on the screen at $P$ depends on $\theta$. Show that the angle $\theta$ at which the first-order bright fringe appears is $6.2^\circ$.
Determine the value of $\theta$ at which the second-order bright fringe appears.
On Fig. 4.2, sketch how the intensity $I$ changes with $\theta$ for $\theta$ from $-15^\circ$ to $+15^\circ$.
A polarising filter is inserted into the path of the light beam that is incident on the diffraction grating in Fig. 4.1. The transmission axis of the filter is at $45^\circ$ to the vertical. Suggest how the intensity variation with $\theta$ for the light on the screen would compare with the answer in (b)(iii).