To convince you that these interference effects are real we have ripple tank set up. Two levers run by motors disturb the water surface and function as periodic point sources. We can vary the frequency of these sources by turning a knob and since the wave velocity is a fixed property of the water tank varying the frequency is also a way of varying the wavelength.
Qualitatively we see that as I increase the wavelength for fixed source spacing the number of lines of constructive and destructive interference varies.
To do something quantitative as well with this experiment we shall
perform an accurate determination of the wave length for
a specific fixed frequency. I do this by measuring the
angle corresponding to the 
line of constructive interference.
We find (say) 
. Knowing d=2.0 cm we can calculate
the wave length at this particular frequency:
If we want to know the speed of waves in the ripple tank
we need to determine the source frequency and then use
Since we have control over the source we know the source frequency
which in this case is 
. The wave velocity is thus
