# Doppler.pdf - The Doppler Effect NAME SECTION I...

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The Doppler Effect 1 | P a g e The Doppler Effect NAME:___________________________________________ SECTION:_________________ I. INTRODUCTION: The Doppler Effect is the apparent shortening or lengthening of wavelength caused by the relative motion between the wave source and an observer. All types of waves experience this effect. In sound waves, the Doppler Effect causes the pitch to sound higher when either the observer or sound source are moving towards each other. Conversely, when the sound source or the observer are moving away from each other, the Doppler Effect causes the pitch of the source to sound lower. In the exact same way, the Doppler Effect works with light waves. The wavelength, or color, of light emitted by a source changes if it is moving towards, or away, from Earth. The Doppler Effect is used by astronomers to study the motions of stars, supernovae, nebulas, galaxies and many other objects, relative to the Earth, and relative to each other. For example, examine Figure 1 in considering a pair of speakers emitting sound waves of the same wavelength (and thus the same pitch). Here, two speakers emit a pulse every second, which travel a certain distance towards an observer before the next pulse is emitted. The wavelength is the distance between these consecutive pulses, and appears different to the observer if the source is moving towards them. On the left hand side these pulses are depicted for a stationary speaker, and on the right side for a speaker moving towards the observer. The moving speaker still emits the pulse every second, but after one second the speaker has moved a short distance forward before emitting the second pulse. Since the speaker moved forward, the distance between pulses is less. Even though the moving speaker is emitting a sound wave at the same rate as the stationary speaker, the motion towards the observer makes the observed wavelength (λ') appear smaller than the emitted wavelength (λ) . Therefore, for the observer the pulses appear closer together. Figure 1
The Doppler Effect 2 | P a g e We observe this phenomenon in our daily life. A well-known example is that of a passing train. As the train approaches you, the pitch of the train gets higher (this is because the pitch, or frequency, is inversely related to the wavelength of the sound wave, thus smaller wavelengths mean a higher pitch). Conversely, if the speaker is moving away from you the opposite happens. Examine Figure 2 in which there is a stationary speaker on the left, and on the right a speaker moving away from the observer. Even though the speaker emits sound at the same rate, as the speaker moves away the wavelength appears to be longer (and the pitch sounds lower). In our example with the train, this is an obvious effect once the train passes you. The sound of the train and its whistle drop drastically in pitch the instant the train passes you!