17.7 The Doppler Effect
3 min read•june 24, 2024
Ever noticed how a siren sounds different as it zooms by? That's the in action. It's all about how change when the source or listener is moving, making the seem higher or lower.
This cool phenomenon isn't just for sirens. It helps astronomers figure out how fast galaxies are moving and even plays a part in creating sonic booms. Understanding the opens up a whole world of sound and motion.
The Doppler Effect
Doppler effect on sound frequency
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- Change in frequency of sound waves perceived by an observer when the source of the sound is moving relative to the observer
- Source and observer moving towards each other:
- Observed frequency higher than emitted frequency (ambulance siren approaching)
- appears shorter to the observer
- Perceived pitch increases due to compressed
- Source and observer moving away from each other:
- Observed frequency lower than emitted frequency (ambulance siren receding)
- appears longer to the observer
- Magnitude of frequency change depends on between source and observer (faster motion, greater frequency change)
- Applies to all types of waves, including sound waves (sirens) and electromagnetic waves (light from galaxies)
Calculations with Doppler shift equation
- relates observed frequency to emitted frequency and relative velocities of source and observer
- General form of equation:
- observed frequency
- source frequency
- speed of sound in (air)
- Signs (+ or -) depend on relative motion between source and observer
- Source moving towards stationary observer: (train whistle approaching)
- Observer moving towards stationary source: (moving car approaching stationary warning bell)
- Both source and observer moving:
- Towards each other: (two trains approaching each other)
- Away from each other: (two trains moving apart)
Real-world applications of Doppler effect
- Sirens on emergency vehicles:
- Vehicle approaches, siren's pitch sounds higher due to Doppler effect (police car approaching)
- Vehicle passes and moves away, siren's pitch sounds lower (fire truck receding)
- Sonic booms:
- Object (aircraft) moves faster than speed of sound, creates
- heard as loud "boom" when it reaches observer
- Doppler effect causes sound waves to pile up in front of moving object, creating (supersonic jet)
- Red shifts in astronomy:
- Light from distant galaxies appears redder than expected due to Doppler effect
- occurs because galaxies are moving away from Earth due to expansion of universe
- Wavelength of light is stretched (redshifted) as galaxies recede, making them appear redder (Andromeda galaxy)
- Amount of redshift used to calculate galaxy's distance and velocity relative to Earth ()
Key Terms to Review (35)
$f_o$: $f_o$ is the observed frequency of a wave, which is the frequency measured by an observer who is moving relative to the source of the wave. This term is particularly important in the context of the Doppler effect, which describes the change in observed frequency or wavelength of a wave due to the relative motion between the source and the observer.
$f_s$: $f_s$ is the frequency of the sound source in the Doppler effect, which is the apparent change in the frequency of a wave (or other periodic event) for an observer moving relative to the source of the waves. The Doppler effect is observed with all types of waves, including sound waves, light waves, and radio waves.
$v_o$: $v_o$ is the initial velocity, or the velocity of an object at the start of a motion or event. It is a fundamental concept in physics, particularly in the study of kinematics and the Doppler effect, as it describes the initial state of an object's motion before any forces or changes act upon it.
$v_s$: $v_s$ is the speed of the source, which is a key parameter in the Doppler effect. The Doppler effect describes the change in the observed frequency or wavelength of a wave due to the relative motion between the source and the observer.
Amplitude: Amplitude is the maximum displacement of a point on a wave from its equilibrium position. It is a measure of the energy carried by the wave.
Amplitude: Amplitude is the maximum displacement or extent of a periodic motion, such as a wave or an oscillation, from its equilibrium position. It represents the magnitude or size of the motion and is a fundamental characteristic of various physical phenomena described in the topics of 1.7 Solving Problems in Physics, 8.4 Potential Energy Diagrams and Stability, 15.1 Simple Harmonic Motion, and beyond.
Blueshift: Blueshift refers to the phenomenon where light from an object moving toward an observer is shifted to shorter wavelengths, causing the light to appear bluer. This effect is a key aspect of the Doppler Effect, which explains how the frequency of waves changes due to the relative motion between the source and the observer. In astronomy, blueshift is particularly significant when analyzing light from stars or galaxies that are moving closer to Earth.
Christian Doppler: Christian Doppler was an Austrian physicist best known for his work on the phenomenon that bears his name, the Doppler Effect. This effect describes how the frequency of waves, such as sound or light, changes based on the relative motion of the source and the observer. Doppler's discoveries laid the groundwork for various applications in fields like astronomy, radar, and medical imaging, highlighting the importance of motion in wave behavior.
Doppler effect: The Doppler effect is the change in frequency or wavelength of a wave in relation to an observer who is moving relative to the wave source. It is commonly observed with sound waves, where the pitch changes as the source and observer move towards or away from each other.
Doppler Effect: The Doppler effect is the change in the observed frequency or wavelength of a wave due to the relative motion between the source and the observer. This phenomenon is observed in various forms of wave propagation, including sound waves and electromagnetic waves.
Doppler shift: The Doppler shift is the change in frequency or wavelength of a wave in relation to an observer who is moving relative to the wave source. It is commonly observed with sound waves and electromagnetic waves such as light.
Doppler Shift Equation: The Doppler shift equation describes the change in the observed frequency or wavelength of a wave due to the relative motion between the source and the observer. This principle is fundamental to understanding the Doppler effect, which has applications in various fields, including astronomy, radar, and medical imaging.
Frequency shift: Frequency shift refers to the change in frequency of a wave in relation to an observer moving relative to the source of the wave. This phenomenon occurs due to the Doppler effect, which explains how sound, light, and other waves can appear to have different frequencies based on the relative motion between the source and the observer. The frequency shift can indicate whether the source is moving towards or away from the observer, affecting how the wave is perceived.
Hubble's law: Hubble's law states that the recessional velocity of galaxies is directly proportional to their distance from Earth, indicating that the universe is expanding. This relationship was first observed by Edwin Hubble in the 1920s and provides crucial evidence for the Big Bang theory, suggesting that galaxies are moving away from each other as space itself expands.
Hz: Hertz (Hz) is the unit of frequency, representing the number of cycles per second of a periodic phenomenon. This term is crucial in understanding wave phenomena, such as how fast a wave oscillates or how sound waves change due to motion. In practical terms, Hertz helps quantify the rate at which events occur in various physical contexts, linking frequency to wave behavior and sound perception.
Longitudinal Waves: Longitudinal waves are a type of wave in which the displacement of the medium is parallel to the direction of wave propagation. This means the particles in the medium oscillate back and forth in the same direction as the wave is traveling.
M/s: m/s, or meters per second, is a unit of measurement that expresses the rate of change in position or the speed of an object over time. It is a fundamental unit in physics that is commonly used to describe the velocity or wave speed of various phenomena.
Medium: A medium, in the context of physics, refers to the material or substance through which a wave or disturbance propagates. It is the physical environment that allows for the transmission of energy in the form of waves.
Observer velocity: Observer velocity refers to the speed and direction of an observer in relation to a source of waves, typically sound or light. This concept is crucial in understanding how the frequency and wavelength of waves change depending on the motion of the observer relative to the source, which directly ties into phenomena such as redshift and blueshift in light and the Doppler Effect for sound waves.
Pitch: Pitch is the perceived frequency of a sound, determining how high or low it sounds to the listener. It is a fundamental attribute of musical notes and is influenced by the frequency of vibrations produced by sound sources. The concept of pitch is critical in understanding music theory and sound waves, as well as how sounds are perceived in different contexts.
Red shift: Red shift is the phenomenon where the wavelength of light or other electromagnetic radiation from an object increases as it moves away from the observer. This results in a shift toward the red end of the spectrum.
Redshift: Redshift is the phenomenon where the wavelength of light or other electromagnetic radiation from a source increases as the source moves away from the observer. This shift towards longer, redder wavelengths is a consequence of the Doppler effect.
Relative Motion: Relative motion refers to the motion of an object as observed from a specific frame of reference. It describes the change in position of an object compared to another object or point in space, rather than in an absolute sense.
Shock wave: A shock wave is a type of propagating disturbance that moves faster than the local speed of sound in the medium. It is characterized by an abrupt, nearly discontinuous change in pressure, temperature, and density of the medium.
Shock Wave: A shock wave is a type of propagating disturbance that travels through a medium, such as a gas, liquid, or solid, at a speed greater than the local speed of sound in that medium. It is characterized by an abrupt, nearly discontinuous change in the pressure, density, and temperature of the medium.
Sonic boom: A sonic boom is a loud explosive noise caused by the shock waves created when an object travels through the air faster than the speed of sound. These shock waves produce a sudden change in pressure, resulting in a characteristic 'boom' sound.
Sonic Boom: A sonic boom is the loud sound caused by the shock waves created when an object, such as an aircraft, travels through the air faster than the speed of sound. It is a phenomenon associated with the Doppler effect and shock waves, and occurs when the object's speed exceeds the speed of sound.
Sound Waves: Sound waves are the vibrations that travel through a medium, such as air or water, and carry energy from one location to another. These waves are created by the oscillation of particles in the medium, which causes the pressure and density of the medium to fluctuate, resulting in the propagation of the sound wave.
Source Velocity: Source velocity refers to the speed at which a source of waves, such as sound or electromagnetic radiation, is moving relative to an observer. This concept is particularly important in the context of the Doppler effect, which describes the shift in the observed frequency or wavelength of a wave due to the relative motion between the source and the observer.
Tuning fork: A tuning fork is a two-pronged metal device that produces a fixed pitch when struck. It is used to study sound waves and resonance phenomena.
Tuning Fork: A tuning fork is a two-pronged, U-shaped metal device that, when struck, vibrates at a specific frequency and produces a pure musical tone. This vibrating device is commonly used in various scientific and musical applications, particularly in the context of understanding wave phenomena such as beats and the Doppler effect.
V = fλ: The equation v = fλ, known as the wave equation, describes the relationship between the speed (v) of a wave, its frequency (f), and its wavelength (λ). This equation is a fundamental principle in the study of wave phenomena, including sound waves and electromagnetic waves like light.
Wave propagation: Wave propagation refers to the way waves travel through a medium, transferring energy from one location to another without the permanent displacement of particles. This concept is essential in understanding how sound, light, and other types of waves behave when they move through different environments. The characteristics of wave propagation, such as speed and direction, can be affected by factors like frequency and the properties of the medium through which they travel.
Wavelength: Wavelength is the distance between successive crests or troughs of a wave. It is typically represented by the Greek letter lambda ($\lambda$).
Wavelength: Wavelength is a fundamental characteristic of waves, representing the distance between consecutive peaks or troughs of a wave. It is a crucial parameter that describes the spatial properties of various wave phenomena, including light, sound, and other types of oscillations.