As shown in the figure, light is passed through two slits and shines on a screen. The path of the light to a position on the screen is different for the two slits, and depends upon the angle θ the path makes with the screen. If we suppose the screen is far enough from the slits (that is, "s" is large compared to the slit separation "d") then the paths are nearly parallel, and the path difference is simply "d" sin θ. Accordingly, the condition for constructive interference is: