The doppler effect
The Doppler effect is observed whenever the source of waves is moving with respect to an observer. The Doppler Effect can be described as the effect created by a moving source of waves in which there is an apparent upward shift in frequency for observers towards whom the source is approaching and an apparent downward shift in frequency for observers from whom the source is receding. It is important to note that the effect does not result because of an actual change in the frequency of the source. The Doppler effect can be observed for any type of wave - sound wave, water wave, light wave, and much more. We are most familiar with the Doppler effect because of our experiences with sound waves. Maybe you recall an instance in which an emergency vehicle or police car that was traveling towards you on the highway, for example. As the car approached with its siren blasting, the pitch of the siren sound was high, and then suddenly after the car passed by, the pitch of the siren sound was low. That was the Doppler effect - an apparent shift in frequency for a sound wave created by a moving source.
|
ExampleSuppose that there is a happy bug in the center of a circular water puddle. The bug is shaking its legs in order to create the disturbance that travels through the water. If these disturbances originate at a point, then they would travel outward from that point in any and all directions. Since each disturbance is traveling in the same medium, they would all travel in every direction at the same speed. The pattern created by the bug's shaking would be a series of concentric circles as shown in the diagram above. These circles would reach the edges of the water puddle at the same frequency. An observer at point A would observe the disturbances to strike the puddle's edge at the same frequency that would be observed by an observer at point B. The frequency at which disturbances reach the edge of the puddle would be the same as the frequency at which the bug produces the disturbances. If the bug creates disturbances at a frequency of 2 per second, then each observer would observe them approaching at a frequency of 2 per second.
|
ExampleNow suppose that our bug is moving to the right across the puddle of water and producing disturbances at the same frequency of 2 disturbances per second. Since the bug is moving towards the right, each consecutive disturbance originates from a position that is farther from observer A and closer to observer B. Each consecutive disturbance has a shorter distance to travel before reaching observer B and that takes less time to reach observer B. Observer B observes that the frequency of the arrival of the disturbances is higher than the frequency at which disturbances are produced. On the other hand, each consecutive disturbance has a further distance to travel before reaching observer A. For this reason, observer A observes a frequency of arrival that is less than the frequency at which the disturbances are produced. The net effect of the motion of the bug is that the observer towards whom the bug is moving observes a frequency that is higher than 2 disturbances per second. The observer away from whom the bug is moving observes a frequency that is less than 2 disturbances per seconds. This effect is known as the Doppler Effect.
|