Question

Here is a qualitative question about waves. You can figure out the answers by examining the relevant formulas on the formula sheet.

A) For periodic waves of a fixed velocity (light in air, sound in air at a particular temperature, ripples on water of a certain depth), should the frequency be increased or decreased to produce waves of longer wavelength?

Answer 1

For a periodic wave, velocity of the wave is given by

where of the wave.

If the velocity is fixed, then the product of must remain the same.

So for producing wave of longer wavelength, the frequency must be decreased, so that the product remains the same.

For eg, for a sound wave of frequency 220 Hz having speed of 440 m/s, has a wavelegth of 2 m. If the wavelength is increased to 4 m, the frequency decreases to 110 Hz while speed is kept constant.

Hence, keeping the velocity constant, in order to produce waves of longer wavelength, the frequency should be decreased.