Interference of water waves
When two water waves meet, some spots surge while others stay calm.
Water waves from two sources with the same frequency, vibrating in step, interfere where they meet. At points whose path difference to the two sources is a whole number of wavelengths, the two waves always add in phase and the vibration is reinforced; at points where the path difference is an odd number of half-wavelengths, the waves are out of phase and cancel, and the vibration is weakened. Regions of reinforcement and cancellation alternate and stay fixed in place; each follows a hyperbola with the two sources as its foci.
Resultant amplitude along a distant line
Taken along a line parallel to the line joining the sources, 3 m away; peaks mark constructive interference and zeros mark destructive interference.
Think about it
With the sources 1.8 m apart and a wavelength of 0.6 m, how many lines of constructive interference lie between the sources?
The path difference can be 0, ±0.6 or ±1.2 m (it must be less than the separation of 1.8 m), giving 5 lines of constructive interference. Change the wavelength or the separation and count the solid lines.
Model assumptionsSources at the same frequency and in phase · Amplitude decays slowly with distance · Top view of the water
Data log
Set the parameters and click “Record data”. Record several rows with different parameters to plot their relationship and fit a straight line; pause and record at different times to study how a reading changes with time.