Understanding
The Doppler Effect
And its Applications
Doppler Effect
We experience Doppler effect every day. However, we may not realize that what we experience is because of Doppler effect.
Often, we notice that when an emergency vehicle, with a blaring siren, approaches us the loudness and the pitch of the sound keep increasing. The moment it has passed us, the pitch and loudness seem to be decreasing quickly. Why is that?
It is because of Doppler effect or Doppler shift. It is a wave phenomenon. In 1842, the Austrian physicist Christian Doppler described this phenomenon.
This phenomenon occurs not only in sound but also in electromagnetic waves – light.
Let us first understand what Doppler effect is and then explore its applications in various fields such as astronomy, medicine and radar.
Wave Front From A
Stationary Vehicle
In order to understand the Doppler effect, we need to review how sound waves propagate in a medium like air.
Let us consider a case of a stationary emergency truck which has the siren turned on. Assume that the siren produces sound waves with a constant wavelength of λ. A person, who is standing on the right side of the truck at a distance, will hear the siren with the same wavelength λ as the siren produces.
In fact, a person who is standing on the left side of the truck too hears the siren with the same wavelength λ.
But the picture changes when the emergency vehicle starts moving.
Wave Front From An
Approaching Vehicle
Now let us consider the case of the emergency vehicle in motion.
Assume that the emergency vehicle moves towards the stationary person with a constant speed.
As it approaches the stationary person, the wavefront successively start bunching together because of the motion of the vehicle and consequently the wavelength of the sound wave start shrinking, as the picture above illustrates. Consequently, the stationary-person, who is standing on the right side, hears the siren with decreasing wavelength, in other words, with increasing tone or frequency.
Wave Front From A
Receding Vehicle
What happens when the vehicle crosses the stationary person and starts receding from that person?
As the picture above shows, the stationary persons hears the sound with increasing wavelength or decreasing frequency and consequently of lowering pitch.
Conclusion
In conclusion, the Doppler effect or the Doppler shift is a wave phenomenon that causes the increasing loudness or tone as a sound source approaches a stationary person and also causes the decreasing loudness or tone when the sound source recedes from the same stationary person.
It should be noted that this phenomenon occurs not only in sound waves but also in light waves (electromagnetic waves). In fact, it is a wave phenomenon.
The same Doppler effect is the cause for the red-shift (increasing wavelength) occurring in the light-spectrum from distant stars or galaxies that are moving away from us.
Blue-shift (decreasing wavelength) can occur if the light source is moving towards us. For instance, the light-spectrum from the Andromeda galaxy is blue-shifted because it is moving towards our Milky Way galaxy.
Applications
Doppler effect is an effective tool, and it is widely applied in various fields. Some of them are listed below.
Radar: to detect the speed of an approaching or a receding vehicle.
Astronomy: Doppler effect not only occurs with sound but also happens with light. Using the red-shift observed from the light spectrum of galaxies, Edwin Hubble, in 1929, proposed the expansion of the Universe. And it resulted in the modern theory for the origin of cosmos: The Big Bang theory.
Medical: Ultrasonography, based on Doppler effect, is an effective tool to detect several issues related to the flow of blood in organs. For example, the velocity measurement helps with diagnosis of stenosis in veins and arteries.
References
- Britannica, The Editors of Encyclopedia. “Doppler effect”. Encyclopedia Britannica, 14 Aug. 2024, https://www.britannica.com/science/Doppler-effect. Accessed 21 August 2024.
- https://imagine.gsfc.nasa.gov/features/yba/M31_velocity/spectrum/doppler_more.html
- https://www.grc.nasa.gov/www/k-12/airplane/doppler.html
- https://en.wikipedia.org/wiki/Doppler_effect