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AQA A-level Physics Interference Required Practical 2 Mark Scheme
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Mark scheme for the corresponding document: 'AQA A-level Physics Interference RP 2'.
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Q1.
(a) Measurement of at least 30 fringe widths
(check that candidate has not miscounted) e.g. 30 fringe widths = 40 mm or 41 mm.
Correct answer of 1.3 or 1.4 mm quoted to 2sf with unit ✔(one mark)
OR
Correct answer 1.33 – 1.37 mm to 3sf with correct unit ✔✔ (two marks).
If candidate quotes value in range 1.33 – 1.37 mm to 3sf
they achieve both the 2nd and 3rd marks (A quote to 3sf is
justified in terms of uncertainty if a large number of fringe
widths have been measured).
For 2nd & 3rd marks allow ecf from incorrect measurement
in 1st mark.
If the printing process in your centre alters the scale of
this diagram, measure the values on your printed
question papers and mark the scripts accordingly. Send
details to the moderator.
If a candidate is visually impaired and using a modified
paper that alters the scale of this diagram, measure the
values on the printed question paper and mark the script
accordingly.
3
(b) 1 mark for intermediate step where candidate doesn’t get correct final answer. i.e.
calculating % uncertainty of total measurement (i.e. % uncertainty in w) ✔
OR for both marks:
Uncertainty in w = ± 0.03 mm ✔✔
(Full 2 marks for correct answer with unit – No unit no mark unless a correct
intermediate step has been completed which will have been credited for 1 mark as
explained above)
Uncertainty in measurement of multiple fringes is ± 1 mm
(precision of ruler used).
E.g. for length 41 mm ± 1 mm % uncertainty = 1/41 x 100 =
2.4 %
Uncertainty in w (single fringe)
= 2.4 x 1.4/100 = ±0.03 mm
Simply quoting 0.03 - NO marks
No penalty for omission of ±
2
(c) (i) (Using w = 1.40 mm)
Wavelength = 5.60 × 10–7 m ✔
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Allow ecf for value of w from (b)
Consistent unit required for the mark. No sf penalty.
1
(ii) (Intermediate step) % uncertainty = 5.8% ✔
(From %uncty s = 3.3%, w = 2.4%, D = 0.1% % uncertainty
in wavelength = 3.3 +2.4+0.1 = 5.8%).
Allow ecf from (b)
1
(iii) (Using wavelength = 5.60 × 10–7 m)
Uncertainty in wavelength = ± 3.2 × 10–8 (m) ✔ or ± 32 (nm) or ± 3.2 × 10–5
(mm)
Allow ecf from (c)(i) & (ii)
No sf penalty
If the value is consistent with the wavelength quoted in (c)(i),
allow the numerical answer without the unit, otherwise a
unit is required.
1
[8]
Q2.
(a) w from ✔
3 sf answer ✔
2
(b) 1 ✔
2 ✔
allow ecf in 2 ✔ for wrong w but not for POT error
2
(c) 1 ✔
allow ecf for wrong d in 1 ✔
2 ✔
number of fringes seen = 13 (integer only) 3 ✔
allow 12 or 14 fringes
3
(d) close jaws using ratchet ✔
confirm that instrument reads zero ✔
2
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