Stand on the sidewalk and listen to the sound of a car as it approaches and passes by. You will notice that the pitch of the sound is higher as the car approaches and then becomes lower as it moves away. That change in pitch is the result of the Doppler Effect.
How does it happen? Sound waves move outward from the source of the sound though the air in all directions. The pitch of the sound results from the spacing in the sound waves. If the waves are closer together, the sound has a higher pitch. If they are farther apart, a lower pitch. This spacing between waves, or the distance from the crest of one wave to the next, is called the wavelength.
Let’s look at two different situations. The first diagram below represents a car that is stopped at a light. We’ll concentrate on a single sound, perhaps the hum of the engine. Each circle represents a crest of the sound wave moving outward.
If we were to examine the same car a little later, each circle would be bigger, but distance between each circle would remain the same. As the wavelength of the sound is the same in all directions, anyone that is stationary relative to the car will hear the same pitch.
In the second diagram below, the car is moving to the right. Because the source of the sound wave (the car) moves between the times when two wave crests leave the source, the wave crests end up closer together in the direction of motion and farther apart in the opposite direction.
So, someone standing at point A will hear a higher pitch and someone standing at point B will hear a lower pitch. Since the pitch that they hear depends on the car’s speed, they could find out how fast the car is moving by measuring the shift in the sound’s wavelength.
The Doppler Effect also works for light. In the case of light, different wavelengths of light are different colors. Blue light has short wavelengths and red light has long wavelengths. If an object is moving toward us, particular colors of light given off by that object have a shorter wavelength than they do when stationary and we say the light is “blue shifted.” Likewise, if an object is moving away from us, particular colors of light given off by the object have a longer wavelength than they do when stationary and we say the light is “red shifted.” By carefully measuring the apparent shift in the wavelength of the light, astronomers can determine how fast an object is moving toward or away from us.