scanning electron microscopy (SEM)
/ SKAN-ing ee-LEK-tron my-KROS-koh-pee /
Imagine exploring a dark statue with a thin flashlight beam, sweeping it back and forth across the surface and noting how much light bounces back from each spot — gradually building a picture of the statue's shape. Scanning electron microscopy works on the same idea: instead of sending electrons through the sample, it sweeps a fine beam of electrons across the outside surface and detects what bounces or scatters off, point by point. The result is a striking, three-dimensional-looking image of a surface.
In an SEM, the focused electron beam scans across the specimen in a tight raster pattern, like an old television building an image line by line. Where the beam hits, it knocks loose low-energy 'secondary' electrons; a detector counts how many escape from each point, and bright spots are mapped where many electrons emerge. Because more electrons escape from edges and ridges, the final image has a deep sense of depth and texture. The sample is usually coated with a whisper-thin layer of gold or platinum so its surface conducts and gives off electrons well.
SEM is the tool of choice for seeing the outer shape and texture of cells and structures: the spiky surface of a pollen grain, the bumpy outside of a red blood cell, the bristling hairs on an insect's leg. Its trade-offs are the mirror image of TEM: SEM shows surfaces beautifully in three dimensions but cannot peer inside, its resolution is somewhat coarser than TEM's, and like all electron microscopy it requires dead, dried, often metal-coated samples in a vacuum — so again, never a living cell, and the colours are always added later.
An SEM image of a human red blood cell shows its smooth, dimpled disc shape in vivid three-dimensional relief — the famous biconcave 'doughnut without the hole' surface.
SEM reveals surface shape in three dimensions, but cannot see inside the cell.
SEM shows the outside surface only, not the interior; the dramatic three-dimensional look is real, but the colours are false-coloured afterward, and the cells are dead and metal-coated.