Doppler Effect Doppler effect- an observable change in frequency. As a moving object approaches an observer the frequency goes up, as is moves away the frequency goes down. The wavelength gets smaller as it approaches and gets larger as it goes away. A shock wave is produced when the speed of sound is passed.

Which direction is the source moving?

Which direction is the source moving? To the left! Notice the compression of wavelengths in the front and the spreading of them behind. Remember: Larger frequency = smaller wavelength!

f o is the frequency observed. f s is the actual frequency v is the velocity of waves in the medium (Sound or light) v s is the velocity of the source relative to the medium v o is the velocity of the observer relative to the medium f o = f s (( 1 ± (v o /v))/ ( 1 ± (v s /v))

Example: A horn on a car with a frequency of 410 Hz is being sounded. The car is moving toward you at 25 m/s the speed of sound is 331 m/s. What frequency do you hear?

Example: A horn on a car with a frequency of 410 Hz is being sounded. The car is moving toward you at 25 m/s the speed of sound is 330 m/s. What frequency do you hear? f o = f s (( 1 ± (v o /v))/ ( 1 ± (v s /v)) f o = 410 Hz ((1 + (0/331 m/s))/ (1- (25/331 m/s) F o = Hz Note: the observer is not moving hence the 0 for v o. The source is coming toward you thus the “-” is used in the denominator. Notice the math If the observer is coming toward the source the top is “+” if the source is approaching the observer then the bottom is negative. Reverse the signs for the opposite happening.

A Red shift means it is moving away. A blue shift it is coming toward you. Redshift of spectral lines in the optical spectrum of a supercluster of distant galaxies (right), as compared to that of the Sun (left).

When we look at the spectrum of the sun we get a red shift on one side and a blue shift on the other side. What does this tell us?