The key difference between doppler effect in sound and light is in their velocity. For the doppler effect in sound, the velocity of the observer and source are relative to the medium in which the waves go through are important, whereas for the doppler effect in light, only the relative difference in velocity between the observer and the source are important.
Doppler effect or Doppler shift is the change in frequency of a wave relative to an observer who is moving relative to the wave source. This effect was named after the physicist Christian Doppler. The major reason for the Doppler effect to occur is the emission of each successive wave crest from a position closer to the observer (compared to the crest of the previous wave) when the source of the waves is moving towards the observer. This makes each wave take slightly less time to reach the observer compared to the previous wave. Therefore, the time taken by the arrivals of successive wave crests at the observer’s end reduces, increasing the frequency. This leads to the waves bunching together.
CONTENTS
1. Overview and Key Difference
2. What is Doppler Effect in Sound
3. What is Doppler Effect in Light
4. Doppler Effect in Sound vs Light in Tabular Form
5. Summary – Doppler Effect in Sound vs Light
What is Doppler Effect in Sound?
Doppler effect in sound is the change in the frequency of the sound observed by an observer due to the velocity of the observer and the source of the sound, which are relative to the medium in which the sound passes through. Sound waves cannot pass through vacuums; the sound requires a medium to pass through. Therefore, the velocity of the wave of sound through the medium we are using (usually air surrounding us) affects the Doppler effect.
Generally, the speed of the sound source and the receiver relative to the medium is comparatively lower than the velocity of the sound waves in the medium. Therefore, we can use the following equation for the calculations.
Where f is the frequency (observed), f0 is emitted frequency, c is the speed of waves in the medium, vr is the speed of observer relative to the medium, and vs is the speed of sound source relative to the medium.
There are several applications of the doppler effect of sound, including acoustic Doppler current profiler, siren, medical applications such as echocardiograms, Leslie speaker, etc.
What is Doppler Effect in Light?
Doppler effect in light is the apparent change in the frequency of the light observed by an observer due to the relative motion between the observer and the source of light. Light is a type of electromagnetic wave that does not require a medium to pass through. Therefore, we can consider that light is passing through a vacuum. For the waves passing through a vacuum, the Doppler effect depends only on the relative speed of the observer and the light source.
For example, we can describe the phenomena of redshift and blue shift using the Doppler effect. When considering visible light, when the light source is moving away from the observer, it causes the frequency received by the observer to be lower than the frequency transmitted by the light source. This is named the redshift. Moreover, if the light source moves towards the observer, the frequency received by the observer becomes greater than the transmitted frequency. Then the frequency of light shifts towards the high-frequency end of the visible light range, which leads to the blue shift.
What is the Difference Between Doppler Effect in Sound and Light?
Sound waves propagate through a medium while light does not require a medium to pass through. Therefore, the key difference between doppler effect in sound and light is that for the doppler effect in sound, the velocity of the observer and the source are relative to the medium in which the waves go through are important, whereas for the doppler effect in light, only the relative difference in velocity between the observer and the source are important.
The below infographic lists the difference between doppler effect in sound and light in tabular form.
Summary – Doppler Effect in Sound vs Light
Sound waves propagate through a medium, while light does not require a medium to pass through. Therefore, for the doppler effect in sound, the velocity of the observer and the source are relative to the medium in which the waves go through are important, whereas for the doppler effect in light, only the relative difference in velocity between the observer and the source are important. Thus, this is the key difference between doppler effect in sound and light.
Reference:
1. Millis, John P. “What is Blueshift?” ThoughtCo.
Image Courtesy:
1. “Velocity0 70c” By en:TxAlien – en:Image:Velocity0 70c.jpg (CC BY-SA 3.0) via Commons Wikimedia