(a)[3]
State the meaning of diffraction and the meaning of interference.
(b)[3]
Light from source $S_1$ falls on a diffraction grating, as shown in Fig. 6.1. The light has a single frequency of $7.06 \times 10^{14}\,\text{Hz}$. The diffraction grating has $650$ lines per millimetre. Calculate how many orders of diffracted light the grating produces. Do not count the zero order. Show your working.
(c)[1]
A second source $S_2$ is used instead of $S_1$. The light from $S_2$ has a single frequency lower than that of the light from $S_1$. State and explain whether more orders are seen with the light from $S_2$.