Interference In the preceding section, you learned that when light passes through a slit, the expression for the intensity 7 of the light on a screen is given by Equation 1. Recall here that a is the slit width, 2 is the wavelength of the light, and is the angle defined in the drawing below. If two such slits are separated by a small distance d from each other, then not only will there be diffraction at each slit, there will also be interference between the diffraction patterns of each slit. The pattern observed in this case may be referred to as a double-slit, diffraction-interference pattern. The diagram below shows how the interference pattern (often called Young's interference) arises. R Figure 9.4: Young's interference Two rays, labeled 1 and 2, are shown for illustration. If the path difference between the two rays is a multiple of the wavelength, then the waves interfere constructively, and one obtains a maximum in the interference pattern. If the path difference is a half-integer multiple of the wavelength, then the waves interfere destructively, and one obtains a minimum in the pattern. The two equations describing constructive and destructive interference, respectively, are given and d sin 0 = m2; m = 0, 1, 2, . d sin 8 = (m + =)2); m = 0,1, 2,... screen position, and the result is Again, the Huygens-Fresnel principle may be used to calculate the intensity as a function of the (5) (6) where is given by B = (nd /2) sin 0 / = Im(cos