2.1 Electron microscopy
Light microscopes cannot be used to study very small structures (i.e. those less than approximately 0.2 µm across), because there is a limit to the resolution, or 'resolving power' of such microscopes. The limiting factor that prevents small structures being discriminated using a light microscope is the wavelength of visible light.
The problem of the limited resolution of the traditional light microscope was overcome in the 1930s, by the development of electron microscopes, which use beams of electrons instead of light. Electron beams have a shorter wavelength than that of visible light, and the wavelength of a beam of electrons decreases as the velocity (speed) at which they travel increases. Electron beams accelerated at high velocity through an appropriately prepared tissue section and focused on a fluorescent screen can hence be used to resolve cellular components that are as small as 1 nm across.
There are two different types of electron microscopy. Scanning electron microscopy (SEM) is a technique used to study the surface of intact cells and tissues. The sample is coated with a thin metallic layer that deflects an electron beam onto a detector. The internal organisation of cells is studied using the second technique, transmission electron microscopy (TEM), in which a beam of electrons is passed through a very thin tissue section, allowing the analysis of cell organelles and other components.