wave path which angle with respect to a straight line parallel to the optical axis is close to 0, this approximation condition can be further simplified as where L is the distance between two planes along the optical axis. Due to the fact that an incident wave on a diffracting plane is effectively a plane wave if where L is the distance between the diffracting plane and the point wave source is satisfied, Fraunhofer condition is where L is the smaller of the two distances, one is between the diffracting plane and the plane of observation and the other is between the diffracting plane and the point wave source. Focal plane of a positive lens as the far field plane In the far field, propagation paths for wavelets from every point on an aperture to a point of observation are approximately parallel, and a positive lens (focusing lens) focuses parallel rays toward the lens to a point on the focal plane (the focus point position on the focal plane depends on the angle of the parallel rays with respect to the optical axis). So, if a positive lens with a sufficiently long focal length (so that differences between electric field orientations for wavelets can be ignored at the focus) is placed after an aperture, then the lens practically makes the Fraunhofer diffraction pattern of the aperture on its focal plane as the parallel rays meet each other at the focus. Examples In each of these examples, the aperture is illuminated by a monochromatic plane wave at normal incidence. Diffraction by a narrow rectangular slit The width of the slit is . The Fraunhofer diffraction pattern is shown in the image together with a plot of the intensity vs. angle . The pattern has maximum intensity at , and a series of peaks of decreasing intensity. Most of the diffracted light falls between the first minima. The angle, , subtended by these two minima is given by: Thus, the smaller the aperture, the larger the angle subtended by the diffraction bands. The size of the central band at a distance is given by For example, when a slit of width 0.5 mm is illuminated by light of wavelength 0.6 μm, and viewed at a distance of 1000 mm, the width of the central band in the diffraction pattern is 2.4 mm. The fringes extend to infinity in the direction since the