Principle of operation of the transmission electron microscope (TEM).

The idea

A TEM uses a beam of accelerated electrons, produced by an electron gun and focused by magnetic lenses, to illuminate a very thin specimen. Because electrons have a much shorter de Broglie wavelength than visible light, the resolution is far better than an optical microscope. Electrons pass through the specimen; regions that are thicker or denser scatter more electrons, so fewer reach the fluorescent screen or detector, forming a magnified image. The whole column is evacuated so electrons are not scattered by air. The specimen must be extremely thin so electrons can transmit through it. Contrast arises from differences in electron scattering, and the image can be recorded digitally or on film.

What earns the marks

• Electrons are emitted from a heated filament or field-emission source and accelerated through a high potential difference, giving them high kinetic energy and a short de Broglie wavelength.
• Magnetic lenses, not glass lenses, focus and magnify the electron beam because glass would absorb or scatter electrons.
• The specimen must be very thin so that a sufficient number of electrons can pass through it rather than being absorbed or scattered.
• Electrons that pass through the specimen are detected on a fluorescent screen or by an electronic detector, producing a magnified image.
• Contrast is produced because thicker or denser regions scatter electrons more, so fewer electrons reach the detector from those areas.
• The column is kept under vacuum to prevent electrons from colliding with gas molecules, which would scatter the beam and blur the image.

Common mistakes and how to put them right

• Thinking that glass lenses are used to focus electrons.

  Put it right: Magnetic lenses are used because glass would absorb or scatter electrons.

• Believing that the image is formed by electrons reflected from the surface.

  Put it right: In a TEM electrons are transmitted through a very thin specimen.

• Assuming that a TEM can examine any solid sample directly.

  Put it right: Specimens must be extremely thin and prepared specially, and the column must be evacuated.

Exam tips

• Link the high resolution explicitly to the short de Broglie wavelength of accelerated electrons, not simply to their high speed.
• Describe the path of electrons in order: source, acceleration, magnetic lenses, thin specimen, detector.
• When comparing with an optical microscope, mention both the wavelength advantage and the need for a vacuum and thin specimens.