slit and illumination also extend to infinity. If , the intensity of the diffracted light does not fall to zero, and if , the diffracted wave is cylindrical. Semi-quantitative analysis of single-slit diffraction We can find the angle at which a first minimum is obtained in the diffracted light by the following reasoning. Consider the light diffracted at an angle where the distance is equal to the wavelength of the illuminating light. The width of the slit is the distance . The component of the wavelet emitted from the point A which is travelling in the direction is in anti-phase with the wave from the point at middle of the slit, so that the net contribution at the angle from these two waves is zero. The same applies to the points just below and , and so on. Therefore, the amplitude of the total wave travelling in the direction is zero. We have: The angle subtended by the first minima on either side of the centre is then, as above: There is no such simple argument to enable us to find the maxima of the diffraction pattern. Single-slit diffraction using Huygens' principle We can develop an expression for the far field of a continuous array of point sources of uniform amplitude and of the same phase. Let the array of length a be parallel to the y axis with its center at the origin as indicated in the figure to the right. Then the differential field is: where . However and integrating from to , where . Integrating we then get Letting where the array length in radians is , then, Diffraction by a rectangular aperture The form of the diffraction pattern given by a rectangular aperture is shown in the figure on the right (or above, in tablet format). There is a central semi-rectangular peak, with a series of horizontal and vertical fringes. The dimensions of the central band are related to the dimensions of the slit by the same relationship as for a single slit so that the larger dimension in the diffracted image corresponds to the smaller dimension in the slit. The spacing of the fringes is also inversely proportional to the slit dimension. If the illuminating beam does not illuminate the whole vertical length of the slit, the spacing of the vertical fringes is determined by the dimensions of the illuminating beam. Close examination of the