By reference to two waves, state: the principle of superposition

1. (a)(i)By reference to two waves, state: the principle of superposition[2]
2. (a)(ii)By reference to two waves, state: what is meant by coherence.[1]
3. (b)Two coherent waves P and Q meet at a point in phase and superpose. Wave P has an amplitude of 1.5cm and intensity I. The resultant intensity at the point where the waves meet is 3I. Calculate the amplitude of wave Q.[2]
4. (c)The apparatus shown in Fig. 5.1 is used to produce an interference pattern on a screen. Light of wavelength 680nm is incident on a double-slit. The slit separation is a. The separation between adjacent fringes is x. Fringes are viewed on a screen at distance D from the double-slit. Distance D is varied from 2.0m to 3.5m. The variation with D of x is shown in Fig. 5.2.
5. (c)(i)Use Fig. 5.2 to determine the slit separation a.[3]
6. (c)(ii)The laser is now replaced by another laser that emits light of a shorter wavelength. On Fig. 5.2, sketch a possible line to show the variation with D of x for the fringes that are now produced.[2]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.