Using two waves as your reference, state the principle of superposition.
With reference to two waves, state what coherence means.
Two coherent waves P and Q arrive at a point in phase and superpose. Wave P has an amplitude of $1.5\,\text{cm}$ and intensity $I$. The combined intensity at the point where the waves meet is $3I$. Calculate the amplitude of wave Q.
The setup shown in Fig. 5.1 is used to form an interference pattern on a screen. Light with wavelength $680\,\text{nm}$ is incident on a double-slit. The slit separation is $a$. The spacing between neighbouring fringes is $x$. Fringes are observed on a screen a distance $D$ from the double-slit.
The distance $D$ is changed from $2.0\,\text{m}$ to $3.5\,\text{m}$. Fig. 5.2 shows how $x$ varies with $D$. Use Fig. 5.2 to find the slit separation $a$.
The laser is now substituted with a different laser that emits light of a shorter wavelength. On Fig. 5.2, sketch a possible line to show how $x$ varies with $D$ for the fringes now formed.