Question 1

What is the Doppler effect?

Accepted Answer

The Doppler effect is the change in observed frequency (or wavelength) of a wave when the source, observer, or both are moving relative to the medium. When they approach each other, the observed frequency increases (higher pitch for sound, blueshift for light). When they move apart, the frequency decreases (lower pitch, redshift).

Question 2

Why does a siren sound different as it passes you?

Accepted Answer

As an ambulance approaches, each successive wave crest is emitted from a position closer to you, compressing the wavelength and raising the frequency (higher pitch). After it passes and moves away, each crest is emitted farther away, stretching the wavelength and lowering the frequency (lower pitch). The siren itself emits a constant frequency throughout.

Question 3

What happens when the source reaches the speed of sound?

Accepted Answer

When a source reaches the speed of sound (Mach 1), wave crests pile up in front of it, creating a shock wave known as a sonic boom. The Doppler formula yields an infinite observed frequency at this point, which physically corresponds to the formation of the shock front. This calculator returns zero to indicate this singularity.

Question 4

Does the Doppler effect apply to light?

Accepted Answer

Yes, but the formula is different for electromagnetic waves because light does not need a medium. The relativistic Doppler formula accounts for time dilation effects. In astronomy, redshift (frequency decrease) indicates galaxies moving away, and blueshift indicates approach. This is key evidence for the expansion of the universe.

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