DOPPLER'S PRINCIPLE. A law in phys his enunciated by Christian Doppler, an Austrian physicist, in 1St?... If a body which is entitling waves of any kind recedes from the instrument tqhiclt is receiving the the wine-iiiinth•r is apparently decreased: and eonversely. if the vi brating body approaches the reeeiving instru ment. the •ave-number is increased. Similar statements can lie made, in general. of the effect of making the receiving instrument move away from or toward the vibrating source. This alter ation in wave number is called 'Doppler's Prin ciple.' having been first states] in 1842 by Prof. Christian Doppler, of Prague. If the natural velocity of waves in the medium connecting the vibrating body and the receiving instrument is V, if the velocity of approach of the vibrating body is r, and if N is the natural wave-nomber of the waves, the new wave-number will be N', where N' 1" = N . ' whereas, if r is the velocity of recession of the vibrating body, N' .
+v If, however, the vibrating body is not moving, but the receiving instrument is receding with a velocity v, then (V— r); and, if v is the velocity of approach of the receiv ing instrument, (V + v).
Illustrations of Doppler's principle are af forded in acoustics and in light. If a sounding body, e.g. a whistling locomotive, is approaching a listener, the pitch of the note heard is higher than when it is at rest or when receding. Simi larly, if a source of light, e.g. a star or one edge of the rotating sun, is approaching an observer rapidly, the color due to any train of waves in the radiation produced in the human eye is not what would he seen if there were no relative mo tion in the line of sight : the color actually seen will he slightly shifted toward the blue portion of the spectrum. If a photographic plate is used to record the radiation, the actual change in wave-number can he measured, and thus an idea obtained of the velocity of the source of light.