The first electron microscope was developed in the 1930s. In a conventional microscope light waves are channeled through glass lenses to magnify the image. However, this is limited by the quality of the lens and by the wavelength of visible light. It was known that an electron beam had waveform characteristics like light. Unlike light, the wavelength could be controlled by adjusting the velocity of the electrons, and the beam could be focused by a magnetic field. This is the basis of electron microscopy and it allows magnifications and resolutions far greater than those of the light microscope.
There are two types of electron microscope. The first was the transmission electron microscope (TEM). Scanning electron microscopes (SEM) became available in 1965. Surface details which would be impossible to see with a light microscope can be magnified up to 300,000 times.
The electron microscope consists of an evacuated cylinder inside which are a tungsten filament (the cathode); a metal plate with a central hole (the anode) and several magnetic lenses. An electric current heats the filament to a temperature at which it emits electrons. A high negative voltage applied to the cathode accelerates electrons towards the anode with some passing through the hole. The resulting beam is focused on the specimen by the first magnetic lens (the condenser).
In a TEM the specimen is an ultra-thin section of material. When electrons hit it most will pass through it. Some electrons bounce off heavy atoms in the specimen. The pattern this forms is translated into an image when the beam is refocused by the second magnetic lens (the objective). The image is magnified by a third lens (the projector) and can be viewed on a screen.
The beam of the scanning microscope passes across the surface of a specimen rather than traveling through it. When the beam hits a conductive surface it is split into its constituent electrons at various energy levels. The signal produced by secondary electrons is detected, amplified and displayed on screen in a way similar to that of a normal television image.
© East Lothian Council 2000
All images in this exhibition are
© Royal Botanic Garden Edinburgh