Let us consider a point source P0 in front of a rectangular slit AB as shown in Fig. 4.1. According to geometrical optics (i.e., under the assumption of the validity of the rectilinear propagation of light), the source will cast a sharp shadow on the screen SS′; thus, the region A′B′ will be uniformly illuminated and there will be complete darkness in the remaining portion of the screen. However, if one carefully observes the region near A′(or B′) then he will find either a gradual variation of intensity or even a fringe pattern. This is due to the phenomenon of diffraction, and because of the smallness of λ (the wavelength of light), these effects cannot be observed unless one makes very careful observations. Indeed, in the limit of λ → 0, the gradual variation of the intensity (or the fringe pattern) will tend to disappear.
KeywordsFocal Plane Gaussian Beam Line Source Spherical Wave Plane Wave Incident
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