Doppler Effect: A Moving Source
What happens when the source of the waves moves toward you, a stationary observer? Again, you encounter more waves per unit time than you did before so the frequency is shifted up. What has changed, in effect, for the waves?
- their speed
- their wavelength
This time the shift occurs because the wavelength has been lowered by the movement of the source.
| The effective wavelength is l
' |
= |
| v
|  |
| f
|
|
-/+ |
| us
| |
| f
|
|
= |
| v -/+ us
| |
| f
|
|
Use the first sign (-) when the source moves toward the observer, and the second sign (+) when it moves away.
The detected frequency is:
| f ' |
= |
| v
| |
| l '
|
|
= f |
(
|
| v
| |
| v -/+ us
|
|
)
|