Doppler Effect for Light
In Section 18-8 we discussed the Doppler effect (a shift in detected frequency) for sound waves traveling in air. For such waves, the Doppler effect depends on two velocities–namely, the velocities of the source and detector with respect to the air. (Air is the medium that transmits the waves.)That is not the situation with light waves, for they (and other electromagnetic waves) require no medium, being able to travel even through vacuum. The Dopplereffect for light waves depends on only one velocity, the relative velocity Inbetween source and detector, as measured from the reference frame of either. Let represent the proper frequency of the source–that is, the frequency that is measured by an observer in the rest frame of the source. Let f represent the frequency detected by an observer moving with velocity Relative to that rest frame. Then, when the
where AA (= 1A – Ao I) is the wavelength Doppler shift of the light source. If the source is moving away from us, A is greater than Ao and the Doppler shift is called a red shift. (The teen does not mean that the detected light is red or even visible; it merely means the wavelength increased.) Similarly, if the source is moving toward us, A is less than Ao and the Doppler shift is called a blue shift.