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Electron Microscopes

An electron microscope is a microscope that uses an electron beam to create an image of the target.

Learning Objective

  • Explain why electron microscopes provide higher resolution than optical microscopes


Key Points

    • Electron microscopes are very useful as they are able to magnify objects to a much higher resolution than optical ones.
    • Higher resolution can be achieved with electron microscopes because the de Broglie wavelengths for electrons are so much smaller than that of visible light.
    • In electron microscopes, electromagnets can be used as magnetic lenses to manipulate electron beams.

Terms

  • CCD

    A charge-coupled device (CCD) is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. The CCD is a major technology required for digital imaging.

  • de Broglie wavelength

    The wavelength of a matter wave is inversely proportional to the momentum of a particle and is called a de Broglie wavelength.


Full Text

We have seen that under certain circumstances particles behave like waves. This idea is used in the electron microscope which is a type that uses electrons to create an image of the target. It has much higher magnification or resolving power than a normal light microscope. It can achieve better than 50 pm resolution and magnifications of up to about 10,000,000 times , whereas ordinary, nonconfocal light microscopes are limited by diffraction to about 200 nm resolution and useful magnifications below 2000 times.

Electron Microscope Image

An image of an ant in a scanning electron microscope.

Let's first review how a regular optical microscope works . A beam of light is shone through a thin target and the image is then magnified and focused using objective and ocular lenses. The amount of light which passes through the target depends on its densities, since the less dense regions allow more light to pass through than the denser regions. This means that the beam of light which is partially transmitted through the target carries information about the inner structure of the target.

Optical and Electron Microscopes

Diagram of the basic components of an optical microscope and an electron microscope.

The original form of electron microscopy, transmission electron microscopy, works in a similar manner using electrons. In the electron microscope, electrons which are emitted by a cathode are formed into a beam using magnetic lenses (usually electromagnets). This electron beam is then passed through a very thin target. Again, the regions in the target with higher densities stop the electrons more easily. So, the number of electrons which pass through the different regions of the target depends on their densities. This means that the partially transmitted beam of electrons carries information about the densities of the inner structure of the target.

The spatial variation in this information (the "image") is then magnified by a series of magnetic lenses and it is recorded by hitting a fluorescent screen, photographic plate, or light-sensitive sensor such as a CCD (charge-coupled device) camera. The image detected by the CCD may be displayed in real time on a monitor or computer.

Electron microscopes are very useful as they are able to magnify objects to a much higher resolution. This is because their de Broglie wavelengths are so much smaller than that of visible light. You hopefully remember that light is diffracted by objects which are separated by a distance of about the same size as the wavelength of the light. This diffraction then prevents you from being able to focus the transmitted light into an image.

Therefore, the sizes at which diffraction occurs for a beam of electrons is much smaller than those for visible light. This is why you can magnify targets to a much higher order of magnification using electrons rather than visible light.

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