![]() How does this affect the size and intensity of the central diffraction band?(b) In what way is diffraction from each slit related to the interference pattern in a double-slit experiment?(c) When a tiny circular obstacle is placed in the path of light from a distant source, a bright spot is seen at the centre of the shadow of the obstacle. There are thus marked out a series of circles, whose radii x are given by x 2 +r = (r- +.Answer the following questions:(a) In a single slit diffraction experiment, the width of the slit is made double the original width. We imagine a wave-front divided o x Q into elementary rings or zones - often named after Huygens, but better after Fresnelby spheres described round P (the point at which the aggregate effect is to be estimated), the first sphere, touching the plane at 0, with a radius equal to PO, and the succeeding spheres with radii increasing at each step by IX. We will now consider in detail the important case in which uniform plane waves are resolved at a surface coincident with a wave-front (OQ). By the principle of superposition the whole effect may be found by integration of the partial effects due to each element of the surface, the other elements remaining at rest. The,ideal surface of resolution may be there regarded as a flexible lamina and we know that, if by forces locally applied every element of the lamina be made to move normally to itself exactly as the air at that place does, the external aerial motion is fully determined. In the application to sound, where we know what we are dealing with, the matter is simple enough in principle, although mathematical difficulties would often stand in the way of the calculations we might wish to make. Any obscurity that may hang over Huygens's principle is due mainly to the indefiniteness of thought and expression which we must be content to put up with if we wish to avoid pledging ourselves as to the character of the vibrations. a surface at which the primary vibrations are in one phase. It is usually convenient to choose as the surface of resolution a wave front, i.e. The wave motion due to any element of the surface is called a secondary wave, and in estimating the total effect regard must be paid to the phases as well as the amplitudes of the components. ![]() If round the origin of waves an ideal closed surface be drawn, the whole action of the waves in the region beyond may be regarded as due to the motion continually propagated across the various elements of this surface. The principle employed in these investigations is due to C. Thanks to Fresnel and his followers, this department of optics is now precisely the one in which the theory has gained its greatest triumphs. In the infancy of the undulatory theory the objection most frequently urged against it was the difficulty of explaining the very existence of shadows. Later investigations by Fraunhofer, Airy and others have greatly widened the field, and under the head of " diffraction " are now usually treated all the effects dependent upon the limitation of a beam of light, as well as those which arise from irregularities of any kind at surfaces through which it is transmitted, or at which it is reflected.Ģ. Young showed that in their formation interference plays an important part, but the complete explanation was reserved for A. When light proceeding from a small source falls upon an opaque object, a shadow is cast upon a screen situated behind the obstacle, and this shadow is found to be bordered by alternations of brightness and darkness, known as " diffraction bands." The phenomena thus presented were described by Grimaldi and by Newton. ![]()
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