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B As Level, Diffraction

  1. May 29, 2017 #1
    http://pmt.physicsandmathstutor.com/download/Physics/A-level/Past-Papers/Edexcel-IAL/Unit-2/January 2015 (IAL) QP - Unit 2 Edexcel Physics A-level.pdf
    This is the link to the paper if you cant see the image
    Q17, part (b) i
    Please explain
    This is what the markscheme and examiners report say
    I need a good explanation mainly the reason of the wavelength being 4cm

    Wavelength approx. 4 cm (Accept specific value from 3.5 cm to 4 cm or a range)
    Max 3 from:
    No detector output when no gap because microwaves reflected by metal
    Little/no detector output when gap very small because there is a large angle of diffraction but most of the wave (energy) is reflected (allow there is little diffraction)
    Max output when gap approx. equal to wavelength because there is maximum diffraction
    Output less/decreases for larger gaps because less/little diffraction when gap (much) larger than wavelength

    The question asked for an explanation of the shape of the graph, but the link to detector output, the value of the y-axis, was often missed entirely. Others described the shape of the graph in great detail, point by point in some cases, but did not mention diffraction at all despite the introduction of the phenomenon in part (a). A surprising number did not even make a clear statement of their estimate of the wavelength. In terms of marks, most scored at least 1, but more than 3 were rarely awarded. The most common marks were for a statement of a reasonable wavelength and for linking maximum output to ‘maximum diffraction’ when the gap size equals the wavelength. The value of zero output for zero gap size was usually ignored. There is little understanding of the situation when the gap size is less than the wavelength and that the angle through which the wave is diffracted remains a maximum while the intensity decreases as the gap gets smaller than the wavelength. When discussing the distribution of values a mark was sometimes awarded for suggesting that it is uneven, but little else of merit was ever seen.

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  3. May 29, 2017 #2


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    What don't you understand about the explanation given?
  4. May 29, 2017 #3
    The part in ms where it says, "there is a large angle of diffraction but most of the wave (energy) is reflected". What angle are they talking about?
  5. May 29, 2017 #4


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    Look at the diagram in part b. Draw a line from the source running straight through the slit. Now draw a line from the slit to the detector. The angle is the angle between the two lines you just drew.
  6. May 29, 2017 #5
    Ohhh right right, thank youuu
    So the point is that even though the angle of diffraction is increasing with smaller gap the intensity of the waves is reducing and most of the waves reflect which is why diffraction doesn't occur.
    Is this concept clear?
  7. May 29, 2017 #6


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    Pretty much.
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