slits occurs, and an intense band is observed. With the aid of Figure 6-8c, the conditions for maximum constructive interference, which result in the other light bands, can be derived. In Figure 6-8c, the angle of diffraction 0 is the angle from the normal, formed by the dotted line extending from a point o, halfway between the slits, to the point of maximum intensity D. The solid lines BD and CD represent the light paths from the slits B and c to this point. Ordinarily, the distance (OE) ̅ is enormous compared to the distance between the slits (BC) ̅. As a consequence, the lines BD, OD, and CD are, for all practical purposes, parallel. Line BF is perpendicular to CD and forms the triangle BCF, which is, to a close approximation, similar to DOE; consequently, the angle CBF is equal to the angle of diffraction 0. We may then write