In Figure l 7.6b, suppose that for some point P on the screen r₁ = 5,002,248.5A and r₂ = 5,002,25 l.5A, where A is the wavelength of the light. The interference at point P is A. Constructive. B. Destructive. C. Something in between.

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FIGURE 17.6 A double-slit interference experiment. (a) The drawing is not to scale: The distance to the screen is actually much greater than the distance between the slits. Viewing screen The two waves overlap as they spread out behind the two slits. The two overlapped waves interfere, resulting in a pattern of light and dark bands on the screen. Incident laser beam (b) 1. A plane wave is incident on the double slit. 3. Constructive interference occurs when r, and 2 differ by a whole number of wavelengths. 2. Waves spread out behind each slit. 12 Top view of the double slit 4. Destructive interference occurs when r and r2 differ by a whole number of wavelengths plus half a wavelength. Front view of screen m=4. m = 3 The bright fringes are labeled by the m = 2 integer m, starting at the central m=1 maximum. Central m=0 m = 1 maximum m= 2 m = 3 m=4