Main equation
The double-slit interference equation is derived from trigonometry and is given by
s ≈ \(\frac {λD}{d}\)
Note
This equation is an approximation and is valid only when the wavelength of the light and the slit separation are much smaller than the distance from the slit to the screen. In a typical demonstration of Young's slits using light, d will be of the order of a millimeter, while D will be of the order of meters.