Doppler effect equation

Explore the Doppler Effect equation for sound and electromagnetic waves, its applications, and an example calculation in this concise article.

The Doppler Effect Equation

The Doppler Effect, named after Austrian physicist Christian Doppler, is a phenomenon observed in the propagation of waves, such as sound or electromagnetic waves, when there is relative motion between the source of the waves and the observer. This effect results in a change in frequency and wavelength of the waves, which can be quantified using the Doppler Effect equation.

Sound Waves

In the context of sound waves, the Doppler Effect causes a change in the perceived frequency (or pitch) of the sound when the source and observer are moving towards or away from each other. The Doppler Effect equation for sound waves is given by:

fo = fs [(v + vo) / (v + vs)]

• fo represents the observed frequency
• fs is the source frequency
• v is the speed of sound in the medium
• vo is the velocity of the observer relative to the medium
• vs is the velocity of the source relative to the medium

Note that a positive velocity indicates motion towards the other party, while a negative velocity implies motion away.

Electromagnetic Waves

The Doppler Effect also applies to electromagnetic waves, such as light. In this context, the effect results in a shift in the observed wavelength or frequency of light, known as redshift (when the source is moving away) or blueshift (when the source is moving towards the observer). The Doppler Effect equation for electromagnetic waves is:

fo = fs [(c + vo) / (c + vs)]

1. fo represents the observed frequency
2. fs is the source frequency
3. c is the speed of light in a vacuum
4. vo is the velocity of the observer relative to the source
5. vs is the velocity of the source relative to the observer

Similar to sound waves, positive velocity values represent motion towards the other party, while negative values indicate motion away.

Applications and Significance

The Doppler Effect equation has numerous applications in various fields, such as astronomy, radar technology, medical imaging, and weather forecasting. By understanding and accurately applying the equation, scientists and engineers can make essential observations, measurements, and predictions, leading to advancements in these fields.

Example of Doppler Effect Calculation

Let’s consider an example involving sound waves. A car is approaching an observer while sounding its horn. The car has a horn with a frequency of 500 Hz and is moving at a speed of 30 m/s towards the observer. The observer is stationary, and the speed of sound in air is approximately 343 m/s. We will use the Doppler Effect equation for sound waves to determine the frequency observed by the listener.

Recall the Doppler Effect equation for sound waves:

fo = fs [(v + vo) / (v + vs)]

Given:

• fs = 500 Hz (source frequency)
• v = 343 m/s (speed of sound in air)
• vo = 0 m/s (observer’s velocity, since they are stationary)
• vs = -30 m/s (source’s velocity, negative since it is moving towards the observer)

Now, we can substitute the given values into the equation:

fo = 500 [(343 + 0) / (343 – 30)]

Calculate the denominator:

343 – 30 = 313 m/s

Now, substitute the result back into the equation:

fo = 500 (343 / 313)

Compute the ratio and multiply by the source frequency:

fo ≈ 500 (1.096)

fo ≈ 547.8 Hz

Therefore, the observed frequency of the horn by the stationary listener is approximately 547.8 Hz, which is higher than the actual frequency of the horn due to the Doppler Effect.

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