= Solution
<Diffraction contrast> arises because <elastic scattering> transfers transmitted <electron> intensity between the direct beam and <Bragg diffraction> beams. A local change in <crystal orientation>, <strain> or thickness changes that transfer. An <objective aperture> selecting the direct beam makes strongly diffracting regions relatively dark in <bright-field transmission electron microscopy>; selecting a diffracted beam reverses the selection in <dark-field transmission electron microscopy>. This is a change in scattering conditions, not necessarily a change in chemical composition.
Three characteristic effects are \b[bend contours, thickness fringes and dislocation contrast]. <Bend contours> follow positions where a gently curved crystal meets a particular <Bragg diffraction> condition. <Thickness fringes> arise from thickness-dependent exchange of intensity between coupled transmitted and diffracted waves. A <dislocation> distorts nearby planes, changing their diffraction condition and producing localized contrast; changing the diffraction vector can suppress particular dislocations. <Moiré fringes> from overlapping lattices provide another acceptable example, but their coarse spacing is a beat spacing rather than an atomic spacing.
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