Interference of waves
If two lots of waves, that is the green and the blue, run into one another that is interference.
The crests add together making a big wave (constructive interference) but where a crest and a trough meet, they cancel out (destructive interference).
If we arrange so that two lots of waves of the same wavelength and about the same amplitude meet each other then we produce a pattern of constructive and destructive interference.
The dots form lines where the waves are large. In between those lines are lines (in green) where the waves cancel out:
This distinctive interference pattern is shown on these water waves. Drawn on top of the wave photo are lines of constructive interference in red (where there are waves) and lines of destructive interference in green (where there are no waves)
Young's Slits experiment
We can use this pattern to calculate the wavelength. This was first done for light in a well known experiment called Young's Slits experiment. If we were to do that today we might set it up like this using a laser beam as a light source:
To calculate the wavelength we would have to measure the distance between the two slits (d) the distance from the slits to the screen (D) and the distance between two points of reinforcement on the screen(y). That is two bright lines. The wavelength would then be: dy/D. Below there is a more complete explanation in the form of a video lesson.