The “light” at the bottom of this glowing crater is all electrons.

What you’re seeing here is a thin layer of glass with a hole chipped in it.  At the bottom of the hole, just out of the field of view, is a layer of silicon.  In the electron beam microscope, glass looks dark, because it’s an insulator that gobbles up electrons or deflects them, rather than reflecting them back to the electron detector.  Silicon, a semiconductor, is comparatively much better at reflecting back electrons, so it looks much brighter, seeming to glow.   (I colorized the picture later)

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