  Cascades of charged particles are created when high-energy cosmic rays enter
the earth's atmosphere: these 'extensive air-showers' are studied to gain
information on the energy spectrum, arrival direction distribution and mass
composition of the particles above 1014 eV where direct observations using
instruments carried by balloons or satellites become impractical. Detection of
light in the visible and ultra-violet ranges of the electromagnetic spectrum
plays a key role in this work, the two processes involved being the emission of
Cherenkov light and the production of fluorescence radiation. In this paper I
will outline some of the history of the discovery of the Cherenkov process and
describe the use to which it has been put in the study of extensive air-showers
at ground level.
