the transmission electron microscope
A light microscope shines light through a thin slide and uses glass lenses to magnify what comes out the other side. A transmission electron microscope (TEM) does the same thing with a beam of electrons instead of light, and magnetic lenses instead of glass. Electrons accelerated through 100 to 300 thousand volts are fired down a tall evacuated column, pass through an ultrathin slice of the material, and are magnified by magnetic coils onto a screen or camera. The reason for all this trouble is resolution: an electron's wavelength is thousands of times shorter than visible light, so a TEM can resolve detail far too fine for any light microscope — down to individual columns of atoms in the best instruments.
The single most important requirement is that the specimen be thin enough for electrons to pass through — typically less than about a hundred nanometres, often under fifty. Making such a slice is a craft in itself: grinding, ion-beam milling, or cutting with a focused ion beam (FIB). A great strength of the TEM is that from the very same region it can give you two complementary things, just by changing which plane the lenses focus on. Focus on the image plane and you get a magnified picture of the structure; focus on the back focal plane and you get the diffraction pattern (as in selected-area diffraction). Imaging itself comes in two flavours: diffraction-contrast imaging, which makes defects like dislocations visible, and phase-contrast high-resolution imaging, which shows lattice fringes and atomic columns.
Modern aberration-corrected instruments reach resolutions below 0.1 nanometre and, combined with attached spectrometers, can map both structure and chemistry atom-column by atom-column. But honesty matters. What a TEM shows is a two-dimensional projection through the whole thickness of the foil, so overlapping features can be misread. The thin foil you examine may not behave like the bulk material — thinning can relax stresses and rearrange defects — and the intense beam can heat, charge, or damage delicate samples. A TEM image is powerful evidence, but it is a slice seen in projection, not the untouched bulk.
To identify a mysterious precipitate in an alloy, an operator takes a bright-field image (the particle stands out dark), then without moving the sample switches to the back focal plane and records a selected-area diffraction pattern from that same particle. The image shows its shape and size; the diffraction pattern identifies its crystal structure — two answers from one spot.
Switching between image plane and back focal plane lets one TEM region yield both a picture and a diffraction pattern.
A TEM image is a projection through the whole foil thickness, not a surface view or a true 3D map — overlapping features add together. And a foil thin enough for electrons may not represent the bulk, since thinning can relax stress and move defects.