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The Doppler effect is the change in the frequency of a wave when perceived by an observer who is in motion with respect to the source emitting the wave . This effect translates into an increase in frequency (and a decrease in wavelength) as the observer approaches the source (or the source approaches the observer) and a decrease in frequency as they move away from each other. other.
We can see this effect every day when we observe the change in pitch in the sound of a car approaching us and then moving away from us, such as in a Formula 1 race. The sound is noticeably higher when the car approaches towards us than when it passes in front and then moves away.
The change in tone that we perceive may be the most palpable example of the Doppler effect in our daily lives. However, this effect does not only apply to sound waves, but to any type of wave, including light waves. For this reason, the Doppler effect is of great importance in astronomy and many other scientific disciplines.
Doppler effect formula
The Doppler effect can be written in the form of a pair of equations that relate the observed frequency or wavelength to that of the source. Its application depends on whether the source of the waves and the observer are moving towards or away from each other.
When the source approaches the observer
In this case, the equation or formula to use is:
In these equations, f obs represents the frequency perceived by the observer; f source is the frequency that the source emits; λ is the wavelength; v is the speed with which the wave propagates in the medium, and v source is the relative speed with which the source approaches the observer.
As can be seen, the equations predict that the frequency perceived by the observer will increase as the speed with which the source approaches increases, while the opposite occurs with the wavelength.
When the source moves away from the observer
These equations are equivalent to the previous ones, with the difference of the sign of the velocity of the source:
All variables are the same as in the previous case. These equations predict that the frequency perceived by the observer will decrease and the wavelength will increase as the speed with which the source recedes increases.
redshift or redshift
Light behaves like an electromagnetic wave that propagates in a vacuum at a constant speed of approximately 300,000 km/s. What determines the color of light is its wavelength or its frequency. Visible light that has a higher frequency or shorter wavelength is a color between blue and violet, while the one with a longer wavelength and, therefore, a lower frequency, is red.
When the Doppler effect occurs when we move away from a light source (or when a light source moves away from us), we perceive that light with a lower frequency than the one that the source is emitting. This variation in frequency causes the color of light that we perceive to be closer to red than it was before in the spectrum of visible light. For this reason, this phenomenon is called shift or redshift.
As can be seen, redshift is of great relevance in astronomy, since its quantification allows us to indirectly determine the speed with which other celestial bodies are moving away from us. This is accomplished by determining the frequency shift in the atomic absorption lines of light from distant stars and nebulae.
It should be noted that the fact that it is called a red shift does not mean that the light itself is red, but rather that its frequency shifted in the direction or sense in which the frequency of the red color is found in the electromagnetic spectrum.
Blue shift or shift
Blueshift is the opposite effect of redshift: it refers to the increase in frequency of a light wave or electromagnetic wave emitted by a source that is getting closer to us.
The effect of displacement or shift to the blue is used, for example, in pistol speedometers that the police use to determine the speed with which a car is moving, in particular those that work with LIDAR technology (system of measurement and detection of objects by laser).
References
- Juano, A. et al (sf). The Doppler effect and the shift to red and blue . Retrieved from https://www.ucm.es/data/cont/docs/136-2015-01-27-El%20efecto%20Doppler.pdf
- Nuñez, O (sf). Doppler effect: red and blue shift . Recovered from https://www.vix.com/es/btg/curiosidades/4424/doppler-effect-shift-toward-red-and-blue-
- Serway, RA, Beichner, RJ, & Jewett, JW (1999). Physics: For Scientists and Engineers (Saunders Golden Sunburst Series) (5th ed .). Philadelphia, PA: Saunders College Pub.
