State what is meant by the principle of superposition.
Two waves with intensities $I$ and $4I$ are superposed. The waves share the same frequency. Determine, in terms of $I$, the greatest possible intensity of the wave formed.
Coherent light with wavelength $550\,\text{nm}$ is incident normally on a double slit with slit separation $0.35\,\text{mm}$. An alternating pattern of bright and dark fringes appears on a screen positioned $1.2\,\text{m}$ from the double slit, as shown in Fig. 4.1. The screen is parallel to the double slit.
Determine the separation between the centres of neighbouring bright fringes on the screen.
The $550\,\text{nm}$ light is replaced by red light of a single frequency. State and explain how, if at all, the distance between the centres of adjacent bright fringes changes.