Loudspeaker (Jan-Aug 1931)

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THE NATURE OF LIGHT by ROBERT D. CLARK, A. P. S. Rectilinear Propagation and Diffraction Huygens failed to explain the rectilinear propagation of light. “Huygens’ conception of the manner in which wave motion was propagated was as follows: He regarded every point on the wave front as the center of a new disturbance. These secondary disturbances, traveling with equal velocity, are enveloped by a surface identical in its properties with the surface from which the secondary disturbances start, and this surface forms the new wave front.’’* Assuming this as true, let us examine the case of a plane wave front AB approaching an opaque screen PY. Q — ' J AB reaches PY and we have the new wave front CP. Considering any point M on this new wave front as an independent source of vibration according to Huygens’ theory, we may draw the trace of the spherical waves that must come from M. B y Corrections on the article, ‘THE NATURE OF LIGHT,” by Robert D. Clark, appearing in the January issue of the “Loudspeaker.” Page 27, left hand column, lines 13 and 16 should have A”B’ instead of A’B’. Page 27, left hand column, 6 lines up from the bottom the sentence should read. Draw XZ, the path WITHOUT refrection, of the ray from any point on AB. Page 27, right hand column, line 1, A A’ should read A A”. In figure 2 there should be a B at the point where the perpendicular from AB at B intersects RS. — Editor. These are in part represented by the arcs with centers M. It appears that we should have illumination in the region behind PY, hence curved light rays. Since the analogy to sound and water waves was much used, and these actually did bend around corners, the wave theory of light had quite a problem before it. Fresnel gave the satisfactory explanation of rectilinear propagation. To do this he made use of Young’s discoveries about interference. /I wave moving left to right. Let us *Wood, R. W., Physical Optics. Thirteen