Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!seismo!think!husc6!news From: news@husc6.HARVARD.EDU (USENET News System) Newsgroups: sci.physics Subject: Cherenkov radiation Message-ID: <422@husc6.HARVARD.EDU> Date: Fri, 17-Oct-86 11:37:37 EDT Article-I.D.: husc6.422 Posted: Fri Oct 17 11:37:37 1986 Date-Received: Fri, 17-Oct-86 22:51:21 EDT References: <224@sri-arpa.ARPA> Paul Schauble writes: Organization: Harvard University, Cambridge Mass Lines: 27 > > Second, what about Cerenkov(sp?) radiation? If the slower than c > apparant velocity is an effect of the induced radiation from electrons, > why should a particle moving through the intervening vacuum cause > radiation? > From: willner@cfa.harvard.edu (Steve Willner) Path: cfa!willner Cherenkov (should be spelled as you have, but with hacek over the C; becomes Ch in English) radiation occurs when a particle exceeds the speed of light in the current medium of travel. It obviously cannot occur for a particle travelling in vacuum, but it does occur for high energy cosmic rays when they enter the Earth's atmosphere. The Smithsonian Astrophysical Observatory operates a telescope on Mt. Hopkins to observe cosmic gamma rays by Cherenkov radiation of secondary particles. That is, the gamma ray enters the atmosphere and hits an atom, ejecting particles at high energy. These in turn hit more atoms, ejecting more particles. If the gamma ray energy is high enough (above 10**15 eV or so, if I remember correctly), the ejected particles will be going faster than the speed of light in air and will radiate a detectable pulse of light by Cherenkov radiation. ("Detectable" means with a 10-meter diameter collecting dish; you'll never see anything with your eye.) Steve Willner Phone 617-495-7123 Bitnet: willner@cfa1 60 Garden St. FTS: 830-7123 UUCP: willner@cfa Cambridge, MA 02138 USA Telex: 921428 satellite cam