9.4 Wave Speed

Notice that a wave crest always advances by one wavelength after every one period? That’s why v=λ/T, or v=fλ.

v=fλ

For the same medium, wave speed is constant. So when frequency goes up, wavelength does down. In the animation below, both waves have the same speed. So doubling the frequency always result in halving of wavelength.

v=fλ

In different medium, the wave speed is different. When a wave crosses from one medium to another, the frequency does not change, but the speed and wavelength do. In the animation below, both waves have the same frequency. The one with the longer wavelength propagates at a faster speed.

For the original 🍏, click <here>

Nothing can out run a light wave. But for other waves, it is possible for the medium to travel faster than the wave. When this happens for water waves, we get the wake. When this happens for sound, we get the sonic boom. When it happens for the slinky, we get a hovering monkey. Watch this video for a dramatic demonstration.

explanation at xmdemo

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