Video Transcript
Which of the following statements is true of scanning electron microscopes but not of
other types of microscopes? (A) Electron diffraction is used in image formation. (B) The internal structure of objects can be imaged. (C) Electrons that pass from the imaged object to the microscope by quantum tunneling
are used in image formation. (D) Electric, magnetic, and electromagnetic lenses are parts of the microscope. (E) Electrons scattered from the imaged object are used in image formation.
In this question, we are being asked which feature of scanning microscopes is not
common to the other types of microscopes used. To do this, let’s recall the different types of scanning microscopes.
There are transmission electron microscopes, hereafter abbreviated as TEM, which
create an image of a sample by comparing the intensities of electrons that pass
through the sample at various angles and wavelengths.
Another type of microscope is the scanning electron microscope, hereafter abbreviated
as SEM. Like TEM, SEM uses accelerated electrons because their short de Broglie’s wavelength
allows resolution of nano-sized features.
Like TEM, SEM fires an electron beam at a sample. Unlike TEM, however, the electron beam used by the SEM deflects off of the sample,
rather than passing through it. The SEM then collects the electrons that are scattered from the sample’s surface to
form an image. This means there are no thickness limitations for SEM samples, whereas TEM samples
can only measure samples around 100 nanometers thick, since they have to pass
through it, which is more difficult for thicker samples. Also, as a consequence of this difference in measuring, an SEM forms an image of the
sample’s surface, while a TEM forms an image from a diffraction pattern based on the
interior structure of the sample.
There is also scanning tunnel microscopy, known as STM. Unlike SEM and TEM, STM does not send electrons to a sample. Rather, the tunneling microscope picks up electrons that escape from the surface of a
sample by the process known as the quantum tunneling effect. Also, unlike SEM and TEM, the resolution of an STM does not depend on the wavelength
of the electron.
This means that, just like SEM, STM forms a direct image of the sample’s surface. So since TEM uses a diffraction pattern that reveals information about the internal
structure of the sample, since the electrons pass through it, it is the only
microscopy technique that can obtain internal information. Neither SEM nor STM can do so.
Now that we’ve reviewed the different types of microscopes, let’s analyze the
possible answers we’ve been given to determine what is true for only scanning
electron microscopes, SEM.
Option (A) says that electron diffraction is used in image formation. We can rule out this answer because this is only a feature of transmission electron
microscopes.
Option (B) says that the internal structure of objects can be imaged. But once again, the only microscope technique that can do this is TEM, as the
electrons pass through the sample. So this isn’t the right answer either.
Option (C) says that electrons that pass from the imaged object to the microscope by
quantum tunneling are used in image formation. This technique is used only in scanning tunneling microscopy, not in SEM. Option (C) is out.
Option (D) suggests that electric, magnetic, and electromagnetic lenses are parts of
the microscope. Recall that it is the transmission electron microscope that uses both electrostatic
lenses and magnetic lenses. Electrons from the electron beam are sent through an electrostatic lens to sped up as
they approach the sample, then the electrons that pass through are directed using a
magnetic lens on their way to a screen where the image can be formed. An SEM does not use electric, magnetic and electromagnetic lenses in its
construction, so (D) is not the correct answer.
The only option left is (E): Electrons scattered from the imaged object are used in
image formation. Recall that an SEM fires an electron beam at a sample. But unlike a TEM, an SEM collects the electrons that are dispersed rather than
passing through the sample. This is the main feature of an SEM. And so answer (E) is true of scanning electron microscopes. The correct answer is option (E).
