From passing emergency services vehicles to observing distant worlds, the Doppler effect is a fascinating and useful phenomenon. Here we will take a look at what it is and discuss some common applications of the effect in real life.
What is the meaning of the term "Doppler effect"?
According to sources like Encyclopedia Brittanica, the Doppler effect is defined as:
"The apparent difference between the frequency at which sound or light waves leave a source and that at which they reach an observer, caused by the relative motion of the observer and the wave source.
This phenomenon is used in astronomical measurements, in Mössbauer effect studies, and in RADAR and modern navigation. It was first described in 1842 by Austrian physicist Christian Doppler."
What is the Doppler effect?
Like anything in life, the best way to understand something is to observe it for real if possible. So, with regards to the Doppler effect, it is something you probably experience every single day — whether you notice it or not.
The classic example is the relative change in the sound you notice when an emergency services vehicle passes by you with its siren on. If you pay attention, you will notice a change in pitch as it approaches and then passes by.
The siren will be perceived at a higher pitch on approach until the siren's source reaches and then goes past you. Once you are "behind" the siren, the pitch drops.
This occurs because any waves emitted by an object source are compressed (wavelength is squashed and frequency increases), relatively speaking, when approaching an observer. In contrast, waves get stretched (again relatively speaking) out as the source travels away from an observer.
The Doppler effect is also the reason behind the phenomenon of sonic booms in supersonic aircraft.
So long as the source, or observing, objects are moving slower than the speed of light (i.e. non-relativistic), the change in frequency can be calculated using the following formulae:
In both of these formulae, c0 is the speed of the wave in a stationary medium (the speed of sound in this case), and the velocity is the radial component of the velocity (the part in a straight line from the observer).
Both these formulas are non-relativistic approximations that are true as long as the velocity of the moving object is much less than the speed of light.
It is important to note that it is the convention that the velocity is always positive if the source is moving away from the observer and vice versa, negative when it is moving towards it.
A similar phenomenon is observed with light too.
The light from stars, when observed from a point of reference, like Earth, actually changes in color depending on the relative movements of the Earth and the distant star. If the star is moving away from Earth, relatively speaking, the light will shift towards the red end of the spectrum.
This is called "redshifting" when talking about the Doppler effect and light. It has, in other words, lower frequency and longer wavelength.
If the Earth and the distant star are approaching each other, the light from the star will be shifted towards the violet end of the light spectrum. In other words, the light has a higher frequency and shorter wavelength.
This is called "blueshifting".
While not actually practically possible, you should theoretically be able to travel fast enough towards a red light to make it appear green.
You can do the same for any other part of the electromagnetic spectrum depending on the relative motion of an observer to an emitting source.
It should be noted that the above diagrams and formulae work equally well for sound and light so long as they are sources are not moving near the speed of light. If the relative velocity between the emission source and observer does approach the speed of light then relativistic effects need to be taken into account.
For this reason, the above formulae would need to be changed.
This phenomenon is not only interesting from a scientific point of view, but it also has some very useful applications in real life. From studying the motions of stars and to search for double stars in the cosmos, to helping predict tomorrow's weather, the Doppler effect is used regularly to further our understanding of nature around us.
What is the difference between the Doppler effect and the Doppler shift?
In essence, nothing. The two terms are used interchangeably.
However, some do distinguish the two as the difference between the observed apparent change in frequency of sound or light to an observer (Doppler effect), and the actual change in relative motion with respect to a medium between both the emission source and observer (Doppler shift).
The medium is something like the air — it's required for sound to propagate in the first place. Light, on the other hand, travels perfectly fine in the vacuum of space.
In practice, the are both essentially the same thing, especially with reference to observing the relative motion of celestial bodies. This is because the Earth is constantly in motion about its axis and in space, as well as, any other objects in space like planets and stars.
What is the Doppler effect used for?
And so, without further ado, here are some ways that the Doppler effect is used in real life. This list is far from exhaustive and is in no particular order.
1. Scientists use the Doppler effect to observe distant stars
The Doppler effect is a very useful tool for astronomers. Stars are constantly emitting electromagnetic waves in all directions that we can observe from here on Earth.
As the star rotates around its center of mass and moves in space, the wavelengths of its EM radiation shift accordingly relative to our position on Earth.
We observe this as very subtle changes in the EM spectrum, notably the visible light portion of it. When the star moves towards us its EM emission wavelengths get compressed and becomes slightly bluer (blueshifts).
When the star moves away from us, its emitted light becomes ever so slightly redder, or redshifts. To observe this effect, astronomers use something called a spectrograph (a prism-like apparatus) that separates out incoming light waves into different colors.
In the star's outer layer, atoms absorb light at specific wavelengths. These can be observed as "missing" by appearing as dark lines in different colors of the sun's emitted spectrum.
These are useful as markers to measure the size of the Doppler shift. If the star is on its own (no planets or other nearby stars) this pattern should remain relatively constant over time.
If there is a companion star around, the gravitational pull of this unseen body will affect the other star's movement at certain points of its orbit. This will produce a noticeable change in the overall pattern of the Doppler shift over time.
2. The Doppler effect is used to find exoplanets
Just like companion stars, the Doppler effect can be used to find, or at least surmise, their presence around a distant star. As these planets are so small, relatively speaking, it is very hard to observe them directly using conventional telescopes.
Even if we could, they are often obscured from view by the overwhelming glare of their parent star.
Any star that has exoplanets will "wobble" ever so slightly about its axis. We can use the Doppler effect to find candidate star systems. However, it should be noted we can only find larger planets akin to Jupiter or bigger using this approach.
The effect will be more subtle than a companion star, but it is useful to determine the planet's orbital period (aka the length of a "year") and the likely shape of its orbit, and also its likely minimum mass.
For smaller exoplanets, like another Earth-sized planet, other methods are required. Specialist apparatus like NASA's Kepler spacecraft, look for drops in a parent sun's emitted radiation as planets move across the surface of their sun.
Called a "transit method", astronomers can calculate the relative drop in brightness of a star and use that data to calculate the size of the body that transited past the sun. We can even work out how far the exoplanet is and infer information about its likely atmospheric composition.
The Doppler effect, if the observing apparatus is sensitive enough, can even be used to observe the likely atmospheric condition of the planet. According to MIT, in 2010 one of their postdoc graduates, Simon Albrecht, was able to discover that color shifts in the light absorbed by the planet indicated that strong winds were likely present within its atmosphere.
To date, over 4,000 exoplanets (as of September 3rd, 2020 NASA announced that we have confirmed 4,276) have been discovered using things like the Doppler effect. There are also thousands of "candidate" exoplanets that are yet to be officially confirmed.
Amazingly, the first exoplanet was discovered over three decades ago during the 1990s. Since then the number has grown exponentially. As our observing apparatus gets more complex and sensitive over time, who knows what will be able to discover about these distant worlds.
3. Laser Doppler Anemometers also make use of the Doppler effect
Anemometers are devices used to measure wind speeds. They come in various forms and were first invented by an Italian artist, Leon Battista Alberti, in 1450 AD.
The most common ones you are probably familiar with are cup anemometers and vane anemometers.
However, there are specialized ones called Laser Doppler anemometers. Technical called Laser Doppler velocimetry, devices consist of:
- An optical emission device, usually a laser
- Some form of receiving device with an optical system including a photodetector and a photomultiplier
- Some form of system for processing signals receiving from the photomultiplier
The laser is split into two parallel beams using a prism, both of which then pass through a lens that makes them converge at a distant focal point. These beams are then picked up by the receiver and multiplied in order to be useful for measuring wind speed.
The Doppler effect is then used to calculate the particles within the air relative velocity as the light beams are scattered prior to reaching the receiver.
The technique can be used in other applications beyond calculating wind speeds. It is often applied, for example, inflow research, automation, medicine, navigation, and for calibrating and other measurement systems.
We'll discuss some of these techniques later.
4. Flow and level sensors also take advantage of the Doppler effect
Broadly a similar technique as used in Laser Doppler Anemometers, the Doppler effect can also be used to measure fluid flow and as level sensors.
This is a well-established technique and is widely used in fluid dynamics to measure moving liquids and gases. It is non-intrusive and is very useful for things involving reversing flow, chemically reacting or high-temperature media, and rotating machinery, and other situations where physical sensors might be difficult or impossible to use.
This technique does, however, require tracer particles in the flow. The technique works by sending a monochromatic laser beam toward a target liquid of gas.
In some circumstances, like in wastewater, the technique relies on any solid particulate or gas bubbles in the liquid.
A receiver then collects and analyses any reflected radiation. By using the principles of the Doppler effect, any change in wavelength of the reflected radiation can be used to work out the target's relative velocity.
4. Some echocardiograms also make use of the Doppler effect
An echocardiogram is a special noninvasive (meaning the procedure doesn't involve puncturing the skin) procedure to assess the heart's function and structures. Typically the procedure involves the use of a transducer (like a microphone) to send out waves of sound at a very high frequency.
When this transducer is placed on the chest at certain locations and angles, the waves travel through the skin and other body tissues to the heart. When the waves hit the heart, they bounce-back or "echo" off the heart's physical structures.
The returning signals are picked up by a receiver that converts them into electronic signals and passes them to a computer to create moving images of the heart's valves and walls.
Echocardiograms come in various forms, but one of them makes use of the Doppler effect to work. Called—funnily enough—a Doppler echocardiogram, this device is widely used in many medical practices around the world.
This Doppler echocardiogram technique is generally used to measure and assess the flow of blood through the heart's chambers and valves.
It's defined as "the amount of blood pumped out with each beat is an indication of the heart's functioning. Also, Doppler can detect abnormal blood flow within the heart, which can indicate a problem with one or more of the heart's four valves, or with the heart's walls."
There is also another technique called Color Doppler echocardiography. An enhanced version of regular Doppler echocardiography, different colors are used to designate the direction of blood flow.
This technique helps simplify the interpretation of the Doppler technique.
5. Pulse-Doppler RADAR also takes advantage of the Doppler effect
Another interesting application of the Doppler effect is for Pulse-Doppler RADAR. Primarily used for weather studies, this technique is so sensitive it can actually detect the motion of rain droplets and the intensity of the precipitation.
Pulse-Doppler RADAR uses dual-polarization RADAR that sends and receives vertical and horizontal pulses.
Such devices can determine the range to a target using pulse-timing techniques and can calculate the target objects velocity using the principles of the Doppler effect.
First developed for military purposes, one of the first applications was for the CIM-10 Bomarc (an Amerian long-range supersonic ramjet missile armed with a W40 nuclear warhead). This missile was designed to destroy entire formations of enemy aircraft midair.
The technology was also widely employed in fighter aircraft during the 1960s. It is also used in air traffic control systems to pick out aircraft from clutter.
Pulse-Doppler RADAR is also the basis of synthetic aperture RADAR that is commonly used in RADAR astronomy, remote sensing, and mapping. The technique has also found applications in healthcare, for fall risk assessment and fall detection, and for nursing and clinical purposes.
6. RADAR speed guns also make use of the Doppler effect
Another interesting application of the Doppler effect is RADAR and RADAR speed guns. By utilizing the principles of the effect, it is possible to measure the velocity of a target object from a distance.
In the case of RADAR speed guns, a RADAR beam is fired at a moving target, like a car, as it approaches or recedes from the RADAR source. They can either be hand-held or vehicle-mounted and measures calculate the speed of a target vehicle by detecting the change in frequency of the returning RADAR signal.
If the vehicle is approaching, the frequency will be higher than the emitted RADAR source, and vice versa if the vehicle is traveling away from the RADAR gun. Using this information it is then possible to calculate the target vehicles' relative speed to the stationary RADAR gun.
These devices are commonly used for speed limit enforcement but more devices make use of LIDAR rather than RADAR. The underlying principle is the same for both types of a RADAR gun, however.
7. Some speakers exploit the Doppler effect too
And finally, another interesting application of the Doppler effect in real life is in some specialist speakers. Leslie speakers combine an amplifier and loudspeaker that are able to project the signal from an electric or electronic instrument and modifies the sound by rotating a baffle chamber (drum) in front of the loudspeakers.
The speakers also include a rotating set of treble horns at the top of the speaker that move in unison with the rotating base drum. This produces a very unique sound.
The speaker was developed by Donald Leslie in the 1930s to provide a speaker for a Hammond organ that better emulated a pipe or theatre organ. A Hammond organ is an electric organ first invented by Laurens Hammond and John M. Hanert in the 1930s.
These speakers use the principles of the Doppler effect by applying an electric motor to turn acoustic horns into the surroundings of a loudspeaker. A similar effect can be generated by rotating a series of horns in front of a treble driver.
These speakers are also commonly used in association with electric guitars, and some other electronic musical instruments. The speaker can be controlled by a musician through either an external switch or pedal that alternatives between slow and fast speed settings known as "chorus" and "tremolo".
And that's a wrap.
As we have seen, the Doppler effect is not only interesting in and of itself, but has some important real-life applications too. You'll never look at a passing vehicle the same way again.