Doppler Effect

The Doppler effect in light waves can be observed by the spectral analysis of light emitted by luminous objects.

The light from a stationary distant object whose chemical composition is known is refracted at a specific band of light on a spectroscope. That band is known as its index of refraction. If the light, instead, appears at another frequency band in the spectroscope, it can be inferred from the Doppler effect that the body is in motion. When the light appears at a higher frequency band, then the body is no longer stationary but moving toward the observer. The Doppler effected light wave is displaced toward the higher frequency band, which is the blue end of the spectroscope. If the known body's light waves appear at a lower frequency band of the spectroscope, towards the red end, then the body is now in motion away from the observer.

With the use of the spectroscope, astronomers have been able to deduce the chemical composition of the stars. The Doppler effect enables them to determine their movements. In our own galaxy, all stars will be shifted either to the blue or red end because of a slight Doppler effect, indicating either a small movement toward or The Doppler effect. Illustration by Hans & Cassidy. Courtesy of Gale Group. away from Earth. In 1923, however, Edwin Hubble, an American astronomer, found that the light from all the galaxies outside our own were shifted so much toward the red as to suggest that they were all speeding away from our own at very great velocities. At the same time he saw that the recession of galaxies nearer to us was much less than those further away.

In 1929, Hubble and Milton Humason established a mathematical relationship that enables astronomers to determine the distance of galaxies by determining the amount of the galaxy's red shifts. This mathematical relationship is known as Hubble's law or Hubble's constant. Hubble's law shows that the greater the velocity of recession, the further away from the earth the galaxy is.

The concept of the expanding universe along with the corollary idea of the "big bang," that is, the instant creation of the universe from a compressed state of matter, owes much of its existence to Hubble's work, which in turn is an important development of the Doppler effect in light waves. While some recent research challenges the red shift phenomenon for galaxies, most astronomers continue to accept Hubble's findings.