Physics for Geologists, Second edition

Electromagnetic radiation 69
analogous to the grating spectrometers of visible light). Different analysing
crystals are used for different ranges.
Radar
Radar ('RAdio Detection And Ranging') was developed for military purposes
in the 1930s and during the Second World War, and has many uses today,
both military and civilian. It is based on the timing of reflections of pulses of
radio waves from the solid surfaces of land, ships and aircraft. These pulses
can be sent at various frequencies, but they are usually in the microwave
range - and they can heat like the domestic microwave. There are several
technical varieties of radar, the nature of the emissions being suited to various
circumstances. Reflections do not have to be from solid objects because
radar is used to track storms, and detect turbulence ahead of aircraft. It is
perhaps better known now for its detection of speeding motorists and its
use in air-traffic control, but important gains in our knowledge of the Earth,
our oceans and continents comes from satellite radar altimetry carried out
by NASA and the Goddard Space Flight Center. The sea-surface topography
map (Figures 3.3 and 3.4) is constructed from radar altimetry, and is due to
the reflection of radar waves from sea water.
When the police measure your speed on the road, they are measuring the
slight change in phase of the reflected ray (the Doppler effect) exactly like
the sound of an aircraft passing low overhead, or the horn of a passing train.
Radio/microwaves will also penetrate the earth for a short distance, and
have been used for shallow exploration of the subsurface for minerals and
for archaeological exploration. In these applications, the wave is scattered
and attenuated, but some of it is reflected from surfaces separating beds of
contrasting physical properties.
Copyright 2002 by Richard E. Chapman