Physics 9702 · AS & A Level · The diffraction grating
The diffraction grating — practice question
A beam of vertically polarised light with wavelength $540\,\text{nm}$ falls normally on a diffraction grating, as illustrated in Fig. 4.1. The grating has a line spacing of $5.0 \times 10^{-6}\,\text{m}$. The transmitted light falls on a circular screen, with the diffraction grating at the circle centre $X$. The central bright fringe appears at point $O$ on the screen and has intensity $I_0$. $P$ is a point on the screen where line $XP$ makes a variable angle $\theta$ with line $XO$. The light intensity $I$ at $P$ changes with $\theta$.
(a)[2]
State the meaning of diffraction of a wave.
(b(i))[2]
Show that the angle $\theta$ at which the first-order bright fringe is produced is $6.2^\circ$.
(b(ii))[1]
Determine the value of $\theta$ at which the second-order bright fringe is formed.
(b(iii))[3]
On Fig. 4.2, sketch how the intensity $I$ varies with $\theta$ for $\theta$ values from $-15^\circ$ to $+15^\circ$.
(c)[2]
A polarising filter is inserted into the 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 $\theta$ on the screen compares with the answer in (b)(iii).
Worked solution & mark scheme
This 10-mark question has a full step-by-step worked solution and mark scheme. One marking point: “wave travels through a gap / aperture” …