This is an image of graphite coated V4 mica in SEI mode at 30 kV. Note the small flake of mica in the middle of the image-- it appears to be transparent, as if the SEM is seeing through the flake. What's going on?
An SEI image is a map of secondary electron emission from the surface, and since secondary electrons are very low energy-- ~ 100 eV-- they have an inelastic mean free path of a nm or less. It's impossible for such low energy electrons to be generated beneath this flake and emerge through the flake for detection.
This effect of apparent thin sample transparency is due to backscattered electrons coming back through the flake and generating secondary electrons as they emerge from the thin section. In interpreting SEI images one should consider that secondary electrons can be generated directly by the primary beam, but also indirectly by backscattered electrons. One way of understanding this phenomena is to mix SEI and BEI images. Another is to look at the same sample at different beam energies.
In the bottom image the same region of the V4 mica sample is imaged at 5 kV. Note that the thin flake on the surface is no longer seemingly "transparent". The backscattered electrons are lower in energy and thus generate fewer secondary electrons as they pass through the thin section.
Also note that at lower energies the edge effects are less noticeable, and there is less evidence of charging on the fractured edges which did not get coated with graphite.