Young's Slit 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

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8.       (i)      wave sources with constant phase difference                                                     B1

          {NB allow “in phase” and ignore reference to frequency/wavelength/amplitude}

  (ii)    S1 and S2 ‘share the same light’ (AW)                                                                B1

          reference to diffraction at the single slit
OR to wavefronts e.g. “same wavefront reaches S1 and S2 (AW)                     B1

  (iii)   Constructive interference occurs at O                                                                 B1
path difference is zero OR waves meet in phase (AW)                                      B1

  (iv)  recall of formula l=ax/D in any valid form (e.g. x = lD/a)                                  C1

          {NB allow undefined symbols provided they match the above as stated in the
spec., otherwise they must be defined}

          correct sub. with consistent units: l = 2 × 10–3 × 0.6 × 10–3 /1.8                       C1
                                                   l = 6.7 × 10–7 m                                                A1

          {NB allow ecf if mm used: i.e 2 marks for 6.7 × 10–1 OR 6.7 × 10–4}

  (v)   ‘fringe separation’ (AW) would DECREASE                                                       B1

          {NB allow “more fringes would be seen”}

          because x µ l (AW)                                                                                            B1

          {NB allow ‘colour change’ arguments for full marks:

          Colour would change                                                    B1;
to a colour closer to the blue end of visible spectrum (AW) B1}

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