2 - The relevant regions of the electromagnetic spectrum

Summary

The extent of the electromagnetic spectrum is too well known to require description here. We shall be chiefly concerned with the so-called ‘optical’ region, of which the boundaries are determined partly by the methods of detection and partly by the methods of dispersing and analysing the radiation. What is common to all parts of the region is the type of optical element and materials of construction of spectroscopic instruments. Beyond the region on the long-wave side, coherent detectors, paraboloidal aerials, waveguides and dipole arrays are used, and on the short-wave (X-ray) side, optical elements other than diffracting crystals and grazing incidence reflectors are generally unknown. Radiation of 100 Å wavelength liberates photoelectrons with more than 100 eV of energy and the appropriate detection methods are those of radiography and nuclear physics.

Broadly we can identify six wavelength divisions appropriate to optical design techniques:

• 50–15 µm. The far infra-red (FIR), where bolometric, superconductor and semiconductor detectors are the chief methods of detection and measurement, and only specialised materials such as selenium, thallium bromide and various polymer resins such as sulphones have the necessary transparency to make useful refracting components. Optical elements may well be polymer plastics of high dielectric constant. Reflecting elements are coated with gold. New FIR-transparent materials with desirable optical properties are appearing all the time.