. Light, a textbook for students who have had one year of physics. nary spectrometer, to focus the spectrum and to col-limate the light. 68. The Doppler principle. Motion of the stars.—An inter-esting application of the spectroscope is to the determinationof the rate at which stars areapproaching or receding fromthe solar system. This is basedupon a principle known as theDoppler effect, which ap-plies not only to light, but alsoto sound or any) other waivephenomenon. If a source ofwaves is approaching an ob-server, the length of the waves Fisure 71which he receives is less, if it is receding f
. Light, a textbook for students who have had one year of physics. nary spectrometer, to focus the spectrum and to col-limate the light. 68. The Doppler principle. Motion of the stars.—An inter-esting application of the spectroscope is to the determinationof the rate at which stars areapproaching or receding fromthe solar system. This is basedupon a principle known as theDoppler effect, which ap-plies not only to light, but alsoto sound or any) other waivephenomenon. If a source ofwaves is approaching an ob-server, the length of the waves Fisure 71which he receives is less, if it is receding from him greater, thanif there is no motion. The reason is easily explained by figure 71. Suppose thatthe source of waves is moving in the direction of the arrow P,with a velocity v, and let V be the velocity of the waves them-selves. Bear in mind that V depends only upon the propertiesof the wave-carrying medium, not at all upon the velocity ofthe body emitting the waves. If that body sends out a crestwhen it is at A, then that crest will expand into a growing. 134 LIGHT circle with A as center, despite the fad that the source movesaway from A in the meanwhile. Let A, B, C, etc., be positionsof the source at instants differing by a period, so that-a crestis started from each of these points. Then at any later instantthe wavefronts will be circles, as shown, but not concentriccircles, the center of each being displaced toward the right fromthe center of the preceding one by the distance the sourcemoves during one period. If A represents the normal wave-length, as it would be if the source were at rest, the period isa Y. and the motion of the source during this time is , the wavelength received by an observer to the left,in the direction of X, will be increased by this amount, whilethat received by an observer to the right, in the direction Y,will be decreased by the same amount. That is, the observedwavelength, to a stationary observer in the line of motion, will
Size: 1695px × 1475px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1920, booksubjectlight, bookyear1921
