the scanning electron microscope
The scanning electron microscope, or SEM, replaces light with a fine beam of electrons and scans it back and forth across a surface like an old TV raster, building an image dot by dot from the signals that come back. Because electrons have a far shorter wavelength than light, the SEM smashes through the optical microscope's 0.2 micrometre resolution wall and can show features a few nanometres across, at magnifications from 10x to 100,000x or more.
The focused electron beam knocks loose low-energy secondary electrons from the top few nanometres of the surface; a detector counts them, and the count sets the brightness of each pixel. Because the surface's tilt controls how many secondaries escape, the picture looks strikingly three-dimensional, with a huge depth of field — everything from a valley to a peak stays in focus. That is why SEM images of fracture surfaces, fibres and particles look like dramatic landscape photos.
The SEM is the everyday tool of fractography (reading a broken surface to find out how it failed — dimples for ductile, flat cleavage facets for brittle, striations for fatigue) and of surface and particle study. Add an EDS detector and the same instrument also tells you the local chemistry. Caveats: samples must survive vacuum and usually need to conduct, so non-conductors are given a thin gold or carbon coating; and the SEM shows the surface, not the interior.
The broken face of a snapped bolt is examined in the SEM at 2000x: a field of tiny cup-shaped dimples confirms ductile overload, while a nearby region of flat, river-patterned facets marks a brittle patch — the two textures pinpoint how and where the crack ran.
A scanned electron beam and secondary electrons give deep-focus, 3D-looking surface images from millimetres down to nanometres.
SEM images look three-dimensional but they are still surface maps — the beam probes only the top few nanometres to micrometres. Don't read an SEM picture as if it showed the interior, and remember non-conductors need a conductive coating or they charge up and glow.