What is Doppler shift?

Short Answer

Doppler shift is the change in the observed frequency or wavelength of a wave when there is relative motion between the source and the observer. If the source moves toward the observer, the observed frequency increases; if it moves away, the observed frequency decreases.

This shift occurs in all types of waves, including sound and light. It explains why sirens change pitch as they pass and why astronomers detect red shift or blue shift in light from moving stars and galaxies.

Detailed Explanation :

Doppler shift

Doppler shift refers to the change in frequency or wavelength of a wave caused by the motion of the source, the observer, or both. It is a direct result of the Doppler effect. When either the source or observer moves toward each other, the observed frequency increases. When they move apart, the frequency decreases. This change is what we call the Doppler shift.

The Doppler shift is not limited to sound waves; it also applies to electromagnetic waves such as light, radio waves, and microwaves. In daily life, it explains the change in pitch of a passing vehicle’s siren. In science, it helps us understand the movement of stars, galaxies, and other celestial bodies.

How Doppler shift occurs

Waves travel at a fixed speed in a given medium. When the distance between the source and observer changes due to motion, the spacing of the wavefronts also changes.

  • When moving closer:
    Wavefronts get compressed → shorter wavelength → higher frequency
  • When moving apart:
    Wavefronts get stretched → longer wavelength → lower frequency

This change in frequency or wavelength is the Doppler shift.

Doppler shift for sound waves

Sound waves need a medium like air or water. If the source or observer moves:

  1. Approaching motion
  • Wavelength decreases
  • Frequency increases
  • Pitch becomes higher
  1. Receding motion
  • Wavelength increases
  • Frequency decreases
  • Pitch becomes lower

The speed of sound does not change; only the observed frequency changes.

Doppler shift for light waves

Light does not need a medium; it travels through space. The Doppler shift for light is observed as:

  • Blue shift → if the light source moves toward the observer
  • Red shift → if the light source moves away

Astronomers use this shift to measure:

  • Speed of stars
  • Movement of galaxies
  • Expansion rate of the universe

The red shift of distant galaxies is strong evidence that the universe is expanding.

Mathematical expression for Doppler shift

When the source or observer moves, the observed frequency  is:

Where:

  •  = observed frequency
  •  = actual frequency
  •  = speed of wave
  •  = speed of observer
  •  = speed of source

Signs depend on whether they move toward or away.

For light waves, a different relativistic formula is used because light travels at very high speed.

Types of Doppler shift

  1. Positive Doppler shift

Occurs when the observed frequency increases.
Example: source approaching or observer moving toward the source.

  1. Negative Doppler shift

Occurs when the observed frequency decreases.
Example: source moving away or observer moving away.

These signs help scientists measure direction and speed.

Examples of Doppler shift in daily life

  1. Sirens of ambulances and police cars

Pitch increases while approaching and decreases while moving away.

  1. Train horn

Sounds sharper as it comes closer and deeper as it leaves.

  1. Passing motorcycle

Whining sound becomes lower as it goes away.

  1. Water waves created by a moving boat

Front waves are compressed; rear waves are stretched.

Scientific applications of Doppler shift

Doppler shift is widely used in science and technology:

  1. Astronomy
  • Measure speed of stars
  • Identify galaxies moving away
  • Determine expansion of the universe (red shift)
  1. Medical Doppler ultrasound

Checks blood flow direction and speed inside the body.

  1. Police radar guns

Measure speed of vehicles on roadways.

  1. Weather radar

Detects movement of clouds, rain, and storms.

  1. Navigation

Ships and submarines use sonar Doppler shift to determine their speed relative to water.

Importance of Doppler shift

Understanding Doppler shift helps:

  • Explain real-life sound variations
  • Detect motion in space
  • Improve medical imaging
  • Ensure safety in traffic systems
  • Analyze storm patterns
  • Study movement of astronomical objects

It is a key concept in wave physics and modern science.

Conclusion

Doppler shift is the change in the observed frequency or wavelength of a wave due to the motion of the source, observer, or both. When they move closer, the observed frequency increases; when they move apart, it decreases. This shift is seen in sound and light waves and is widely used in astronomy, medical ultrasound, weather forecasting, radar systems, and navigation. Doppler shift provides essential information about movement and speed in both everyday life and advanced scientific research.