The maxima for a two-slit diffraction pattern are found at the angles given by: m = dsin [m = 0, 1, 2, 3...] The m = 0 central maximum occurs straight ahead at an angle of 0 For the side fringes, let's focus on the maxima that occur at positive angles (but we'll keep in mind that the pattern is symmetric so an equal number of side maxima will occur at negative angles) Solving for the order number, we get: m = dsin / Let's calculate the wavelength from the wave speed relationship: v = f = c = c/f = (3 10 8 m/s) / (2 10 10 Hz) = 1.5 10 -2 m Let's see what order number we are at when we reach +90: m = (0.05m)(sin90) / (1.5 10 -2 m) = 3.33 This implies that by the time we have reached +90, the detector has passed the m =1 , m=2 and m=3 maxima and is one-third of the way to the m=4 maxima (but does not reach it) Considering all both sides, there is the central maximum plus 3 side maxima on each side for a total of 7 maxima