Diffraction and the Wave Theory of Light
In Chapter 36 we defined diffraction rather loosely as the flaring of light as it emerges from a narrow slit. More than just flaring occurs, however, because the light produces an interference pattern called a dilTraction pattern. For example, when monochromatic light from a distant source (or a laser) passes through a narrow slit and is then
intercepted by a viewing screen, the light produces on the screen a diffraction pattern like that in Fig. 37-1. This pattern consists of a broad and intense (very bright) central maximum and a number of narrower and less intense maxima (called secondary or side maxima) to both sides. In between the maxima are minima.Such a pattern would be totally unexpected in geometrical optics: If light traveled in straight lines as rays, then the slit would allow some of those rays through and they would form a sharp, bright rendition of the slit on the viewing screen. As in Chapter 36, we must conclude that geometrical optics is only an approximation. Diffraction of light is not limited to situations of light passing through a narrow opening (such as a slit or pinhole). It also occurs when light passes an edge, such
as the edges of the razor blade why se diffraction pattern is shown in . ote the lines of maxima and minima that run approximately parallel to the edges, at both the inside edges of the blade and the outside edges. As the light passes. say, the vertical edge at the left, it flares left and right and undergoes interference, producing the pattern along the left edge. The rightmost portion of that pattern actually lies within what would have been the shadow of the blade if geometrical optics prevailed.You encounter a common example of diffraction when you look at a clear blue
sky and see tiny specks and hairlike structures floating in your view. s,as they are called, are produced when light passes the edges of tiny deposits in the vitreous humor, the transparent material filling most of the eyeball. What you are seeing when a floater is in your field of vision is the diffraction pattern produced on the retina by one of these deposits. If you. sight through a pinhole in an otherwise0 opaque sheet so as to make the light entering your eye approximately a plane wave, you can distinguish indiv iduaJ maxima and minima in the patterns.