Elastic scattering: In elastic scattering, the scattered electrons do not lose their energy. The scattering only causes change in the electrons' trajectories. Elastic scattering gives a strong forward peak in a thin specimen.

Inelastic scattering : All scattering processes that result in the loss of energy of the primary electrons fall under inelastic scattering.

Secondary effects: Secondary effects include the phenomena that are brought about by the primary electron beam. The phenomena that we are concerned with here are:

• Secondary electrons: Secondary electrons are ejected from the atoms in the specimen. The term is usually used for the electrons that have energies below 50 eV. Such electrons can therefore include the primary electrons that lose their energies through successive scattering and reach the surface of the specimen. Secondary electrons are produced in abundance and form the basis of the scanning electron microscopy (discussed in the next lecture).
  
  
• Backscattered electrons: The primary electrons that do retain substantial energy before escaping the specimen surface. Back-scattering is a function of the atomic number wherein samples with larger atomic number give brighter signals.
  
  
• Cathodoluminescence: An electron can knock off a valence electron from the colliding atom creating an electron-hole pair. An electron falls back into the hole releasing the excess energy as light
  
  
• X-rays: If an electron is knocked off from the inner shells of the atom, an electron in the higher energy shells can fill the vacancy in the lower energy state. The energy associated with inner electron transitions fall in the X-ray wavelength region.

We are now ready to see how electron microscopes work. Electron microscopes come in two basic designs: scanning electron microscopes and transmission electron microscopes. The two microscopes differ from each other in the electrons that are detected.