A police radar gun points at an approaching car and, a moment later, shows the car's speed. Nothing touched the car.
The gun read a wave. Two big applications of the electromagnetic Doppler shift work this way: radar speed measurement, and the redshift of distant galaxies.
The shift carries information about motion: away shifts received waves toward longer wavelengths; toward shifts them toward shorter ones.
A faster source makes a bigger shift.
First application: a radar gun sends out waves, and a moving object reflects them back.
The reflected waves come back frequency-shifted, and the size of the shift gives the object's speed.
A police radar gun finds a car's speed from the frequency shift of waves reflected off it.
Second application: the light of distant galaxies arrives redshifted.
Redshifted light means sources moving away: the galaxies are receding from us.
An astronomer finds a galaxy's whole line pattern sitting far toward longer wavelengths. What application of the Doppler shift is this, and what does it show?
The displaced pattern is a redshift, so the galaxy is moving away.
Seen across distant galaxies, this redshift is evidence for the expanding universe.
Galaxy redshift: the galaxy is receding, and across many galaxies that is evidence for the expanding universe.
A stadium radar gun measures a fastball's speed. What does the gun actually read?
The gun sends waves at the ball, and the moving ball reflects them back shifted.
The size of the frequency shift gives the ball's speed.
It reads the frequency shift between the waves it sent and the waves reflected back.
The light of distant galaxies arrives redshifted. What does this show?
What does a radar speed gun measure to find a car's speed?
Ulla says: 'The redshift of distant galaxies is evidence that the universe is expanding.' Is Ulla correct?
Astronomers find the light of distant galaxies redshifted. Which conclusion does this evidence support?
A radar gun must report how fast a car is moving. Which reading gives it that?
Which of these instruments uses the Doppler shift of electromagnetic waves?