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Tuesday, March 27, 2018

Young's Answers


1.       (i)      when (two) waves meet/combine/interact/superpose, etc. (at a point)
M1
          there is a change in overall intensity/displacement
allow for A1 mark: (vector) sum/resultant displacement(s)/AW
A1
  (ii)   constant phase difference/relationship (between the waves)
just stating same frequency not sufficient
B1
[3]

  2.     (i)      path difference of nλ for constructive interference
allow waves arrive in phase
M1
          producing either maximum amplitude/intensity or a maximum
A1
          path difference of (2n + 1)λ/2 for destructive interference
allow waves arrive in anti-/out of phase
M1
          producing either minimum amplitude/intensity or a minimum
A1
max 3 marks; max 1 mark for two correct marking points but with n omitted
  (ii)   x = λD/a = 0.030 × 5.0/0.20
C1
          = 0.75 (m)
give 1 mark max for 0.75 mm but zero for 750 m
A1
  (iii)  1       intensity increases by factor of 4
B1
         position unchanged
B1
2       intensity unchanged
B1
         distance apart of maxima is doubled
B1
3       intensity unchanged
B1
         maxima move to positions of minima (and vice versa)
B1
[11]



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]

  4.     (i)      (wave sources have) constant phase difference (WTTE)                                  B1
{do not allow “in phase” but accept “same phase difference”}
(ii)     difference in length between detector and each wave source (WTTE)                 B1
[2]

  5.     (i)      1.      path diff. = nl (where n = 0,1,2 etc) {allow 0, OR l, OR 2l etc}              B1
2.      path diff = (n + ½)l (where n = 0,1,etc) {allow = 0.5l OR,1.5l, etc}        B1
         {do not allow answers purely about phase diff. e.g. with degrees or
p used and no ref to l}
(ii)     recall of formula l = ax/D                                                                                C1
correct substitution for a, l and D: e.g. ´ = (4.86 ´ 10–7 ´ 2)/0.5 ´ 10–3              C1
x = 1.94 ´ 10–3 m (1.9 or 1.944)                                                                       A1
(iii)    central white fringe                                                                                        B1
other fringes are coloured (WTTE: e.g. allow spectrum formed)                        B1
[7]

  6.     (a)     (i)      evidence of good practice: i.e distance for nx measured e.g.
           5x = 18mm                                                                                 C1
             x = 3.6 mm (OR 3.5 OR 3.7)                                                      A1
           {x = 3.4, 3.8, 3.9, 4.0, or 4 mm, implying ´ is directly
measured, and score 1 mark)
(ii)     for O path difference = 0                                                                         B1
for A path difference = 3(l)                                                                    B1
for B path difference = 1.5(l)                                                                  B1
  (b)   recall of l= ax/D OR ´ =lD/a OR ´ µ l                                                           B1
l is smaller for blue light (than red light) hence ´ is SMALLER (WTTE)           B1
[7]



7.       (i)      constructive interference/waves in phase for maxima
OR destructive interference/waves ‘out of phase’                                              C1
maxima produced when path difference is 0 OR nl (WTTE)                             A1
minima produced when path difference is (n+1/2) l (WTTE)                             A1
          NB answers that do not account for SERIES of both maxima and
minima can score maximum of 2 marks only)
(ii)     recall of x = lD/a                                                                                            C1
          {expressed in any form; allow unusual symbols if correctly identified}
          correct substitution: x = (3.0 ´ 50)/6                                                                 A1
                     x = 25 cm                                                                          A1
(iii)    microwaves vibrate/oscillate/displaced in one plane (WTTE)                             B1
{do not allow travel/propagate in one plane)
signal decreases to zero (WTTE)                                                                      B1
[8]

  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}
[10]



9.       constructive interference: valid diagram and/or explanation: e.g.
when waves (from coherent sources) meet in phase (or nl path diff.)                           B1
waves reinforce: resultant has increased displacement/amplitude                                 B1
correctly shown on diagram or stated
destructive interference: valid diagram and/or explanation: e.g.
when waves meet in antiphase/180° phase diff. {or (n + 1/2) l path diff.}                    B1
waves cancel: resultant has reduced displacement/amplitude                                        B1
correctly shown on diagram or stated
[4]

  10.   diagram:
laser OR light source and single-slit in front of double slit                                           B1
screen (WTTE) (or travelling microscope) behind double-slit                                      B1
(if ‘screen’ is not labelled mark can be obtained by reference to ‘screen’ in text)
          measurements:
measure distance between double-slit and screen                                                        B1
measure distance between neighbouring dark/bright images                                         B1
(allow ‘fringe spacing’ or measure distance for n fringes)
          formula:
recall of l = ax/D                                                                                                      B1
ALL symbols correctly defined
a = distance between slits                                                                              }
x = fringe separation (WTTE)                                                                       } B1
D = distance between slits and screen                                                            }
          (If candidate uses their own symbols they must be used correctly to
score the formula recall mark)
(do not penalise careless use of d and D: i.e. being interposed)
[6]

  11.   (a)     DIFFRACTION                                                                                              B1
(b)     a constant phase difference/relation (WTTE)                                                     B1
(allow “zero phase difference” and “in phase”)
  (c)    constructive interference produces bright lines AND destructive for dark lines    B1
in phase for bright AND antiphase (allow ‘out of phase’) for dark                      B1
(ALLOW diagrams showing crests/troughs meeting crests/troughs for this mark)
path difference = whole number of wavelengths {allow nl} for bright                B1
(ALLOW path difference = l but NOT path difference = ZERO)
path difference = odd number of half wavelengths {allow (n + 1/2) l} for dark   B1
(ALLOW path difference = ½l)
  (d)   recall of l = ax/D (in any valid form)                                                               C1
valid substitution: e.g. ´ = (6.5 × 10 –7´ 1.5)/0.25 ´ 10–3                                    C1
x = 3.9 ´ 10–3 m                                                                                              A1
(3.9 × 10–6 scores 2 marks)
[9]