1. The problem statement, all variables and given/known data When you shine a light on the ripples in a tank you get dark and bright patches like this: http://img687.imageshack.us/img687/4343/rippletank.jpg [Broken] So my question is: how do we count the number of waves? In my experiment I am trying to find the wavelength by dividing the distance travelled by a train of waves by the number of waves in it. Do we have to count the number of bright fringes or the shadows? 3. The attempt at a solution We are not trying to find the distance from one trough/crest to the next (that would be the wavelength). So, what is "one wave" in this case? How many waves are there in the picture I've posted (I can see 7 bright patches and 7 shadows)?