We have so far studied the light microscopy i.e. the microscopic methods that utilize the electromagnetic radiation; typically UV, visible, and infrared; for studying the biological specimens. Electron microscopes, on the other hand, use electrons for the same purpose. We have seen that a confocal laser scanning microscope allows point-by-point scanning of the sample providing three-dimensional information about the optical features of a specimen. Why do we then need electron microscopes when modern light microscopes have become so powerful! We need them because of their very high resolution. Let us recall the expression given in equation 14.1 for the theoretical resolution of a microscope:

          ...................................................... (14.1)

We have seen in lecture 14 that light microscopy fails to give resolution better than ~0.2 μm. Owing to their much smaller wavelengths, electron microscopes can provide ~2-3 orders of magnitude higher resolution than the light microscopes.

Electrons in microscopy

Louis de Broglie in 1924 theorized that particles have wave-like characteristics. Three years later, electron diffraction experiments carried out independently by ‘Davisson and Germer' and ‘Thomson and Reid' demonstrated the wave behavior of the electrons. Within next five years, the idea to use electrons for microscopy was realized when Knoll and Ruska published the images recorded using electrons. The wavelength of a particle with velocity, v and momentum, p is given by de Broglie equation:

          ......................................................... (17.1)

where,

h is the Planck's constant, m is the mass of the particle, and v is the velocity of the particle