ABSTRACT
The electromagnetic spectrum spans wavelengths of many orders of magnitude from long radio waves (<105 Hz; >1 km) down to those of atomic dimensions (figure 2.1). Several of these radiations may be, and are, used to provide microscope sources for imaging materials, although apart from light in the visible spectrum the wavelengths are not visible to the eye. The various types of radiation addressed in this chapter are shown in table 5.1 together with the associated wavelengths. Also included here are electrons which are addressed in a separate chapter (chapter 2) in view of the large and significant range of techniques that have emerged based upon this particular source. The wavelength of visible light of the order 50 nm places this technique in the mid range of those considered in this chapter. Electromagnetic radiation can be considered to have the properties of a stream of particles, photons, and of waves. The former is the basis of geometrical optics theory which is the approach adopted when discussing lens aberrations, for example, whereas the latter tends to physical optics which is used for discussing, for example, polarising microscopy. Geometrical optics is based upon the familiar fundamental laws (i) light travels in a straight line, (ii) parts of a light beam can be treated as separate entities or rays, (iii) the law of reflection and (iv) the law of refraction (Snell’s Law). These laws emerge as approximations from the electromagnetic wave theory which is the basis of physical optics.
