What is an Electron Microscope ?

Simple definition:

This is possible because a "stream" of electrons can be emitted efficiently from an "electron gun" (consisting of a cathode forming the source of electrons which are then accelerated by an anode) and such a stream, called a "beam" of electrons, has wave-like properties. It therefore behaves in a similar way to light. However, an electron beam has an effective wavelength of less than one namometre (< 1 nm), which is much shorter than the wavelength of visible light, which is in the range 400-700 nm, the exact value depending on the colour of the light.

• The fact that beams of electrons behave like waves makes it possible for them to form images.
• The fact that their effective wavelength is so short enables them to form very high resolution images (compared with those formed by visible light). The power of the electromagnets used to control the path of electrons within an electron microscope enables extremely high magnification images to be formed.

High resolution and high magnification are desirable qualities for microscopes so electron microscopes have been developed and made available in several forms to meet different types of special requirements e.g. to image slices of material, or surfaces, or to minimise specific types of limitations e.g. optical aberrations.

Size of Electron Microscopes:

Comparison: Typical light microscopes, e.g. of the type used in biology classes in schools and colleges, are usually sufficiently light and compact that students can comfortably move them between storage and laboratory workbenches. Electron microscopes are much larger can cannot be casually moved around a laboratory or building. Details of the size and shape depend on the type of electron microscope.

Types of Electron Microscope:

There are two main types of electron microscope, TEM and SEM, as well as other types of electron microscopes, incl.

• Transmission Electron Microscope (TEM) Discussed in AS Biology
• Scanning Electron Microscope (SEM) Discussed in AS Biology
• Scanning Tunneling Microscope (STM)
• Field Emission Transmission Microscope (FE-TEM)
• Reflection Electron Microscope (REM)
• Spin-Polarized Low-Energy Electron Microscope (SPLEEM).

The main difference between TEMs and SEMs is:

1. In the case of TEMs the electron beam is directed through the specimen - with the image plane on the opposite side of the specimen from the cathode (source of electron beam).
2. In the case of SEMs, the electron detector equipment is on the same side of the specimen as the incident electron beam - but at an angle of approx 90 degrees to the incoming beam so that the image is formed by electrons that have been scattered by the specimen as opposed to having passed through it.

See compare TEM vs SEM.

Pictures of Electron Microscopes: