Measuring the Wavelength by Young's Experiment

The distance between the first bright band and central bright band is measured to be x, the distance between the screen and the slits is L, and the separation of the two slits is d.

The two light paths, S1 P and S2 P, differ by one wavelength, S2 N = S2 P - S1 P = l.

Since L is much larger than d, which means that: sinq = l / d = x / L

so the wavelength is given by: l = d ( x / L )