= Solution
This is a conventional transmitted-beam <bright-field transmission electron microscopy> image containing high-resolution <electron phase contrast> and visible lattice fringes, rather than an element-specific chemical map. Relative to the scale bar, the smaller particle has an outer projected diameter of roughly \b[$40\ \mathrm{nm}$]. It is approximately rounded and contains a denser outer rim with several lighter internal regions separated by material. A porous or internally hollow particle is a plausible morphological interpretation; the image is not a simple featureless solid sphere.
Greater projected thickness generally removes more intensity from the transmitted beam, producing darker <mass–thickness contrast>. Different crystallite orientations also scatter intensity differently and give <diffraction contrast>. Local lattice fringes reveal crystalline material and changes of fringe orientation indicate more than one coherent domain. Bright/dark edge fringes also contain defocus-dependent <Fresnel contrast>; their width is not a direct shell-thickness measurement. \b[Neither an exact alumina polymorph nor three-dimensional closed porosity can be uniquely established from this projection alone.] Use calibrated fringe spacings and <electron> <diffraction> for phase identification, tilt-series imaging for void geometry, and a through-focus or dose series to test contrast artefacts.
\Image[/past-exam-of-the-mphil-in-micro-and-nanotechnology-enterprise-course-of-the-university-of-cambridge/2019/paper-1-particle-interpretation.png]
{title=Original generic schematic separating projected porosity, crystallite orientation and edge-contrast interpretations}
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