The Double Slit Equation

The fringes are formed due to interference of light from the two slits.A bright fringe is formed by constructive interference due to the reinforcement of light from one slit by light from the other when they arrive in phase with each other.A dark fringe is formed due to destructive interference - the cancellation of light from one slit by light from the other when they arrive in antiphase (180 ° out of phase) with each other.The fringe separation w is given by:where:D is the distance from the slits to the screenλ is the wavelength of the lightand s is the slit spacing.