y12 Young's slits questions

3.       (a)     (i)      constant phase difference
(allow 1 mark for same phase difference or same frequency/wavelength)   B2

(ii)     path difference = λ/2                                                                                   B1

(b)     (i)      evidence shown that fringe width x = 8.0 mm                                           B1
a = λD/x = 6.4 × 10–7 × 1.5/8.0 × 10–3 = 1.2 × 10–4 m                               C1
(give 2 marks for using x = 4.0 mm giving a = 2.4 × 10–4 m)                    A1

(ii)     maximum intensity when y = 0 AND minima at +4 and –4                        B1
correct repeat distance, i.e. 8.0 mm with at least 2 full cycles drawn       B1

[8]