Observers located anywhere else would see the same thing. The observer, who happens to be located in the direction of the bottom of the image, sees the light waves coming nice and evenly, one wavelength apart. The crests are separated by a distance, λ, where λ is the wavelength. The light waves spread out evenly in all directions, like the ripples from a splash in a pond. The source gives off a series of waves, whose crests we have labeled 1, 2, 3, and 4. In Figure 1a, the light source (S) is at rest with respect to the observer. Observer B, whose line of sight is perpendicular to the source’s motion, sees no change in the waves (and feels left out). Observer C sees the waves stretched out by the motion and sees a redshift. Observer A sees waves compressed by this motion and sees a blueshift (if the waves are light). ![]() Wave crest 1 was emitted when the source was at position S 4, crest 2 at position S 2, and so forth. (b) The source S now moves toward observer A and away from observer C. (a) A source, S, makes waves whose numbered crests (1, 2, 3, and 4) wash over a stationary observer. The general principle, now known as the Doppler effect, is illustrated in Figure 1.įigure 1: Doppler Effect. He then applied what he learned to all waves, including light, and pointed out that if a light source is approaching or receding from the observer, the light waves will be, respectively, crowded more closely together or spread out. In 1842, Christian Doppler first measured the effect of motion on waves by hiring a group of musicians to play on an open railroad car as it was moving along the track. And most objects in the universe do have some motion relative to the Sun. ![]() If a star is moving toward or away from us, its lines will be in a slightly different place in the spectrum from where they would be in a star at rest. There is a complicating factor in learning how to decode the message of starlight, however. Astronomers can learn about the elements in stars and galaxies by decoding the information in their spectral lines. The last two sections introduced you to many new concepts, and we hope that through those, you have seen one major idea emerge.
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