Glossary
Approaching Source
A scenario where the wave source is moving towards the observer, causing the observed frequency to be higher than the rest frequency.
Example:
The high-pitched whine of a jet engine getting louder as it flies overhead indicates an approaching source.
Blueshift
A phenomenon in astronomy where light from a celestial object appears to have a shorter wavelength (higher frequency) due to the object moving towards Earth.
Example:
If a star were moving towards our solar system, its light would exhibit a blueshift.
Doppler effect
The change in the perceived frequency of a wave due to the relative motion between the source of the wave and the observer.
Example:
When a race car speeds past, the pitch of its engine sound changes dramatically due to the Doppler effect.
Frequency
The number of wave cycles that pass a point per unit of time, typically measured in Hertz (Hz).
Example:
A high-pitched sound has a high frequency, while a low-pitched sound has a low frequency.
Negative Doppler shift
A decrease in the observed frequency (observed frequency < rest frequency) that occurs when the source is receding from the observer.
Example:
As a concert speaker truck drives away, the bass notes you hear will exhibit a negative Doppler shift, sounding lower.
Observed Frequency
The frequency of a wave as detected by an observer, which can differ from the rest frequency due to relative motion.
Example:
As a police car approaches, the pitch you hear is the observed frequency, which is higher than the siren's actual frequency.
Observer
The entity or point at which a wave is detected or perceived.
Example:
You are the observer when you hear the sound of a distant thunderstorm.
Positive Doppler shift
An increase in the observed frequency (observed frequency > rest frequency) that occurs when the source is approaching the observer.
Example:
When a car honks its horn while driving towards you, you experience a positive Doppler shift in the horn's sound.
Qualitative understanding
Understanding a concept based on its principles and relationships without requiring precise numerical calculations.
Example:
For the AP Physics 2 exam, you need a qualitative understanding of how changing the speed of a wave source affects the observed frequency, not the exact numerical change.
Receding Source
A scenario where the wave source is moving away from the observer, causing the observed frequency to be lower than the rest frequency.
Example:
The deep, rumbling sound of a train fading into the distance signifies a receding source.
Redshift
A phenomenon in astronomy where light from distant galaxies appears to have a longer wavelength (lower frequency) due to the galaxy moving away from Earth.
Example:
Astronomers observe the redshift of light from distant galaxies, indicating the expansion of the universe.
Relative motion
The motion of one object with respect to another, which is crucial for the Doppler effect to occur.
Example:
A stationary person hears a different pitch from a moving train because of the relative motion between them.
Rest Frequency
The actual frequency of a wave as emitted by its source when there is no relative motion between the source and the observer.
Example:
If a speaker is playing a 440 Hz tone while stationary, 440 Hz is its rest frequency.
Source
The origin point from which a wave is emitted.
Example:
The siren on a fire truck acts as the source of the sound waves.