What is the Doppler Effect?

The Doppler Effect (or Doppler Shift) is the change in frequency (or wavelength) of a wave in relation to an observer who is moving relative to the source of the wave.

It’s commonly experienced with sound waves, but it also applies to electromagnetic waves (like light), water waves, and more.

🔍 How Does It Work?

• When a wave source moves toward an observer, the waves get compressed, resulting in a higher frequency (shorter wavelength). For sound, this means a higher pitch.

• When the source moves away from the observer, the waves get stretched, causing a lower frequency (longer wavelength). For sound, this means a lower pitch.

🎯 Examples

• Ambulance siren: As the ambulance approaches, the siren sounds higher-pitched; as it moves away, the pitch drops.

• Astronomy: Light from stars and galaxies shifts toward the blue end (blue-shift) if moving closer and toward the red end (red-shift) if moving away — this helps determine how celestial objects move.

🔢 Doppler Effect Formula (for sound)

If the source and observer move relative to each other, the observed frequency f′f' is:

f′=f×v+vov−vsf' = f \times \frac{v + v_o}{v - v_s}

Where:

• ff = actual frequency emitted by the source

• vv = speed of waves in the medium (e.g., speed of sound in air)

• vov_o = speed of observer relative to the medium (positive if moving toward the source)

• vsv_s = speed of source relative to the medium (positive if moving away from observer)

🌈 Doppler Effect with Light

For electromagnetic waves (like light), the formula depends on whether the speeds are relativistic (close to the speed of light):

Δλλ=vc\frac{\Delta \lambda}{\lambda} = \frac{v}{c}

Where:

• Δλ\Delta \lambda = change in wavelength

• λ\lambda = original wavelength

• vv = relative speed between source and observer

• cc = speed of light

If the source moves towards you, you see blue shift (wavelength decreases). If it moves away, you see red shift (wavelength increases).

📌 Summary