nλ = 2d sinθ
This is the equation derived by William .H. Bragg and his son William L. Bragg to explain the scattering (diffraction) of X-rays from crystals, and show how the angle of incidence between the beam and the layer (θ) is related to the wavelength of the X-rays (λ) and the separation of the scattering layers (d). n is a positive integer.
The Braggs received the 1915 Nobel Prize for Physics for the work that included this
This is the fundamental theory behind X-Ray crystallography.