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I just dug out my 1st yr Physics (Weber, White & Manning, 1952) and they consistently refer to 'speed of light' as opposed to 'velocity of light'. Four other sources (1941, 1948, ~1965 &1962) have velocity. ----- Original Message ----- From: "David & Alison Webster" <dwebster@glinx.com> To: <NatureNS@chebucto.ns.ca> Sent: Friday, September 21, 2012 7:44 PM Subject: Velocity of light > Dear All, Sept 21, 2012 > The recent discussion about transmission of electricity, reading of > HEAT and reading some of Energy... brings to the forefront a question that > has nagged me for decades. Perhaps someone can clarify. > > In the typical elementary physics text we are told that velocity is a > vector quantity, and to quote one text "velocity... may be defined as the > rate of change of position in a given direction". But "In cases where the > direction of motion does not require to be considered , the term speed is > employed to express the rate of travelling." > > Based on the above I would think that the rate of movement of light > should be called speed, i.e. speed of light but I think it is always > called velocity. > For example, if light from the sun is reflected from two 45o mirrors then > a beam of light could be directed from the earth back to the sun and, the > velocity would then be minus 3 x 10^10 cm/sec. Or if variously scattered > or reflected then the velocity, relative to the initial sun to earth > direction, would always be less than 3 X 10^10 and after several > reflections might be zero. > > Is there some good reason why the speed of light is termed velocity of > light ? Or is it called velocity because the textbook authors forget what > they said in chapter one by the time they write chaper eleven ? (And they > all copy from each other) > Yt, Dave Webster, Kentville
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