Introduction

The Doppler effect is the variation in pitch of an emitted wave as the source moves toward or away from the observer. It was first formally stated by Johann Christian Doppler in 1842. Its effects apply equally to light waves, sound waves, or any other type of wave on the electromagnetic spectrum. With appropriate instrumentation, it can be utilized to calculate the velocities with which sources move. These sources can be anything that emits waves, from a passing police car with its siren on to a space shuttle in the atmosphere to a distant star or galaxy.

Explanation of the Doppler Effect

As an object moves towards an observer, the apparent pitch of a sound increases, and as it moves away from the observer, the pitch decreases. (For visible light, these pitch changes translate into blue and red colors, respectively. For simplicity, this discussion will focus on pitch, though it applies equally to light.) The Doppler effect changes the apparent pitch of sounds emitted from sources in motion relative to an observer. For example, study the diagram at left. To the boy in the teal shirt, the sound waves are approaching with much greater frequency than they approach the boy in the red shirt. This is because they are traveling toward him at the speed of sound, but the source of the waves also has speed of its own, causing the waves to "bunch up." Similarly, the waves are being "spaced out" to the red-shirted boy. Since greater frequency translates into higher pitch, the teal-shirted boy hears the sounds of the car at a greater pitch than the red-shirted boy. If the car were to move past the teal-shirted boy, he would hear the sound steadily increase in pitch until the car passes him, then steadily decrease again until he could hear it no longer.

As stated before, the same applies to visible light. Objects moving toward an observer emit waves of greater frequency, which thus have a blue shift, or a departure from true color that tends toward higher-frequency blue light. Objects moving away from an observer emit waves of lesser frequency, which thus have a red shift, or a departure from true color that tends toward lower-frequency red light. This fact has been used to determine the speeds with which stars, galaxies, and other celestial objects are moving away from earth. This allowed scientists to discover that the universe is expanding and calculate the average rate of expansion.