Have you ever heard the siren of an emergency vehicle going past you? If you have stopped to listen, you could hear that the sound of the siren sounds different from when the emergency vehicle is behind you (going towards you), and when it is in front of you (going away from you). This is a phenomenon known as the doppler effect.
What is the doppler effect?
The doppler effect is the apparent difference between the frequency at which sound or light waves leave a source and that at which they reach an observer. This is caused by relative motion of the observer and the wave source (Britannica, 2024).
In order to make things easier to understand, let us imagine a stationary observer and a source of sound that is able to move towards or away from the observer.
Image that illustrates the doppler effect (Science Ready, n.d.)
When the source of a sound moves towards the stationary observer, the frequency of the sound increases relative to the observer’s position (as shown on the right side of the above image). This causes the higher pitch that you hear when the siren of an emergency vehicle approaches you.
Conversely, when the source of the sound moves away from the stationary observer (as shown on the left side of the image), the frequency of the sound decreases relative to the observer's position, hence causing the observer to hear the sound at a lower pitch.
Interesting application of the doppler effect
Other than sound waves, light waves also get affected by the doppler effect. Something important to note is that visible light is a type of electromagnetic wave, and that different frequencies of light waves causes different colours to appear (BYJUS, n.d.):
From the image above, the graph represents the frequencies of the visible light range, while the axis displays the wavelengths. Focusing on the frequencies, we can tell that the cooler colours, such as violet and blue, have higher frequencies (as shown by the shorter periods of the graph), whereas warmer colours, such as red and orange, have lower frequencies (as shown by the longer periods of the graph).
This leads to interesting observations in space. With the right equipment, astronomers are able to tell whether a star is moving towards or away from Earth by noting the colour of the star when they observe it (Alt Shift X, 2013). From the astronomer’s perspective, when a star moves towards Earth, the star may appear blue (as blue light has a higher frequency), whereas when a star moves away, the star appears red (as red light has a lower frequency). The latter is also commonly known as red shift.
Conclusion
In conclusion, the doppler effect is the reason why we hear sounds differently when the sound is either moving towards or away from us. A change in frequency causes the change in pitch, which leads to us hearing “different” sounds even though the source of the sound is still the same.
Works Cited
Alt Shift X (2013). The Doppler Effect: what does motion do to waves? [online] YouTube. Available at: https://youtu.be/h4OnBYrbCjY?si=n2XlGSjAWKorSTcU [Accessed 27 Jun. 2024].
Britannica, The Editors of Encyclopaedia (2024). Doppler effect. [online] Encyclopedia Britannica. Available at: https://www.britannica.com/science/Doppler-effect [Accessed 27 Jun. 2024]
BYJUS (n.d.). Electromagnetic Spectrum - Definition, Characteristics, Range, Diagram. [online] BYJUS. Available at: https://byjus.com/jee/electromagnetic-spectrum-and-electromagnetic-waves/. [Accessed 28 Jun. 2024]
Science Ready. (n.d.). Doppler’s Effect: Explained with Examples – HSC Physics. [online] Available at: https://scienceready.com.au/pages/dopplers-effect. [Accessed 27 Jun. 2024]
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