4 (a) For a progressive water wave, state what is meant by: (b) Two coherent waves X and Y meet at a point and superpose. The phase difference between the waves at the point is 180°. Wave X has an amplitude of 1.2cm and intensity I. Wave Y has an amplitude of 3.6cm.

1. (a)(i)displacement[1]
2. (a)(ii)amplitude.[1]
3. (b)Calculate, in terms of I, the resultant intensity at the meeting point.[2]
4. (c)(i)Monochromatic light is incident on a diffraction grating. Describe the diffraction of the light waves as they pass through the grating.[2]
5. (c)(ii)A parallel beam of light consists of two wavelengths 540nm and 630nm. The light is incident normally on a diffraction grating. Third-order diffraction maxima are produced for each of the two wavelengths. No higher orders are produced for either wavelength. Determine the smallest possible line spacing d of the diffraction grating.[3]
6. (c)(iii)The beam of light in (c)(ii) is replaced by a beam of blue light incident on the same diffraction grating. State and explain whether a third-order diffraction maximum is produced for this blue light.[2]