$$\require{cancel}$$
In the previous lab, we explored interference of light waves coming through two slits. The two waves emerged from the slits in phase, so difference in distances they traveled accounted for the interference pattern observed. In the reading above, we see that if we use many more slits (a diffraction grating), then we get a much more sharply-defined pattern, with very intense bright fringes separated with the same $$d\sin\theta = m\lambda$$ relationship as for the double-slit, but separated by broad regions of darkness.