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Exercise · Q10

Q.Starting from the definitions of wavelength and period, derive the relation v=fλv=f\lambda connecting a wave's speed, frequency, and wavelength.

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✓ Free question

By definition, the wavelength λ\lambda is the distance the wave pattern must shift, along the direction of propagation, for it to look exactly the same again (one full wavelength); the period TT is the time taken for any one particle -- and hence the whole pattern -- to complete exactly one such cycle.

Since the wave travels a distance of one wavelength λ\lambda in exactly one period TT, its speed (distance travelled per unit time) is:

v=distance travelled in one periodtime for one period=λTv = \frac{\text{distance travelled in one period}}{\text{time for one period}} = \frac{\lambda}{T}

Using the definition of frequency, f=1/Tf=1/T, this becomes:

v=λ×1T=fλv = \lambda\times\frac{1}{T} = f\lambda

✓Final answer

Since the wave advances exactly one wavelength in exactly one period, v=λ/T=fλv=\lambda/T=f\lambda.

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