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 Ms: 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 Er: 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.