An electromagnetic lens is designed to focus the electron beam by using a suitable shaped magnetic field. The lens is a symmetric electromagnet consisting of wire coil, magnetic iron yoke and iron pole pieces. A strong magnetic field is produced by passing a current through the windings. This field acts as a convex lens; converge off axis rays back to focus. Like an optical lens, a magnetic lens is characterized by focal length and can be altered by changing the strength of the current.

(EMR) Electromagnetic radiation consists of an electric and a magnetic field component. All EMR travels in a vacuum at the speed of light. EMR is classified related to the frequency//length of the wave.
An EM wave consists of discrete packets of energy, named photons (quantization). The energy of the photons depends on the frequency of the wave. Planck-Einstein equation:
E = h * f
E (energy); h (Planck's constant); f (frequency)
EMR types include in order of increasing frequency//decreasing wavelength: radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, x-rays and gamma rays. EMR contains energy and momentum, which may be imparted when it interacts with matter.

See Gamma Radiation.

An electron is a negatively charged subatomic particle that orbits the positively charged nucleus of an atom and determines chemical properties. The mass of an electron is around 1/1837 that of the proton.

See also Rutherford-Bohr Atom Model, Beta Particle.

(K-capture) An unstable atom with too many protons in the nucleus, and not enough energy to emit a positron, reaches a stable state in the way, that one proton captured an electron from the atom's inner shell (K-shell) and change to a neutron. A neutrino is emitted from the atoms nucleus by this process. The atomic mass of the atom is unchanged, but the decreased number of protons transformed the atom to a different element.

Electron excitation is the discrete energy storage in an orbital electron. The excitation energy results from the absorption of a photon (photoexcitation) or from the absorption of another electron (electrical excitation). The absorbed energy lifts the electron to a higher energy level. This process ends with electron relaxation.

See also Electron Relaxation.