The Doppler Effect Revision Notes

[ 1 Votes ]

In addition to the revision notes for The Doppler Effect on this page, you can also access the following Optics learning resources for The Doppler Effect

Optics Learning Material
Tutorial IDTitleTutorialVideo
Tutorial
Revision
Notes
Revision
Questions
12.2The Doppler Effect


In these revision notes for The Doppler Effect, we cover the following key points:

  • What happens to the wavelength and frequency of a wave when the source and the receiver are moving in respect to each other?
  • What is the Doppler Effect in waves?
  • How the Doppler Effect is applied in sound waves?
  • How the Doppler Effect is applied in light waves?
  • What is the general equation of the Doppler Effect in waves?

The Doppler Effect Revision Notes

When a wave source is moving in a certain direction, the wavelength decreases in the moving direction and increases in the opposite side. This means the sound frequency also changes compared to the original frequency emitted by the source.

By definition, the effect of frequency change when the source and the detector are in relative motion to each other is known as the Doppler Effect.

There are three possible cases of source and detector movement in respect to each other. They are:

  1. Detector moving, source stationary
  2. Source moving, detector stationary
  3. Both detector and source are moving

When the detector is moving and the source is stationary, we have

f = v + vd/v × f0

for the new frequency when the detector is moving towards the source and

f = v - vd/v × f0

for the new frequency when the detector is moving away from the source.

If the source is moving and the detector is stationary, we have

f = v/v - vs × f0

when the source is moving towards the stationary detector and

f = v/v + vs × f0

when the source is moving away from the stationary detector.

When combining all the above equations, we obtain the general Doppler Effect formula

f = v ± vd/v ± vs × f0

The signs are determined by considering the relative movement of source and receiver in respect to each other.

The Doppler Effect in light waves occurs in a similar way as in sound waves. The only difference is the introduction of a new factor, β = v/c in the formula of the new light frequency, where v is the relative speed of the moving object to the source and c is the speed of light in vacuum (c = 3 × 108 m/s). There are two possible cases in this regard:

1. When source and detector are separating from each other. The formula of frequency for the Doppler Effect in this case is:

f = f0 × √1 - β/1 + β

The new frequency will be lower than the original one. Therefore, a phenomenon known as "red-shift" does occur. It means the detected light frequency will shift towards the red part of visible light spectrum, as red light represents the part of the visible light spectrum with the lowest frequency.

2. When source and detector are approaching each other. In this case, we have:

f = f0 × √1 + β/1 - β

Obviously, the new frequency is higher than the original one. Therefore, a phenomenon known as "blue-shift" does occur. It means the detected light frequency will shift towards the blue part of visible light spectrum as blue light represents a part of the visible light spectrum with high frequency (it should have been violet, but violet belongs to the family of blue light).

Whats next?

Enjoy the "The Doppler Effect" revision notes? People who liked the "The Doppler Effect" revision notes found the following resources useful:

  1. Revision Notes Feedback. Helps other - Leave a rating for this revision notes (see below)
  2. Optics Physics tutorial: The Doppler Effect. Read the The Doppler Effect physics tutorial and build your physics knowledge of Optics
  3. Optics Practice Questions: The Doppler Effect. Test and improve your knowledge of The Doppler Effect with example questins and answers
  4. Check your calculations for Optics questions with our excellent Optics calculators which contain full equations and calculations clearly displayed line by line. See the Optics Calculators by iCalculator™ below.
  5. Continuing learning optics - read our next physics tutorial: Mirrors. Equation of Curved Mirrors. Image Formation in Plane and Curved Mirrors

Help others Learning Physics just like you

[ 1 Votes ]

We hope you found this Physics tutorial "The Doppler Effect" useful. If you did it would be great if you could spare the time to rate this physics tutorial (simply click on the number of stars that match your assessment of this physics learning aide) and/or share on social media, this helps us identify popular tutorials and calculators and expand our free learning resources to support our users around the world have free access to expand their knowledge of physics and other disciplines.

Optics Calculators by iCalculator™