When the electron beam
strikes the sample some of the electrons will interact with the nucleus of the
atom in much the same way a space craft will interact with the gravity of a
planet. The negatively-charged electron will be attracted to the positive
nucleus but if the angle is just right instead of being captured by the
"gravitational pull" of the nucleus it will circle the nucleus and
come back out of the sample without slowing down. These electrons are called
backscattered electrons because they come back out of the sample. Because they
are moving so fast, they travel in straight lines. In order to form an image
with backscattered electrons, a detector is placed in their path. When
they hit the detector a signal is produced which is used to form the TV image. All the elements
have different sized nuclei. As the size of the atom nucleus increases, the
number of backscattered electrons increases. Thus, backscattered electrons can be used to get an image that showed the
different elements present in a sample.
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