State what the diffraction of a wave means.
A laser emits a narrow beam of coherent light with wavelength $632\,\text{nm}$. The beam is normally incident on a diffraction grating, as shown in Fig. 4.1. Light spots are seen on a screen parallel to the grating. The distance between the grating and the screen is $165\,\text{cm}$. The brightest spot is $P$. The spots nearest to $P$ on either side are $X$ and $Y$. $X$ and $Y$ are separated by $76\,\text{cm}$. Calculate the number of lines per metre on the grating.
The grating in (b) is now turned about an axis parallel to the incident laser beam, as shown in Fig. 4.2. State what effect, if any, this rotation will have on the positions of the spots $P$, $X$ and $Y$.
In a different experiment using the apparatus in (b), a student observes that the distances $XP$ and $PY$, as shown in Fig. 4.1, are not the same. Suggest a reason for this difference.