The science of light . Edge. — InFig. 36, O representsthe illuminated aper- a ture and E a straight FlG- 36* edge perpendicular to the plane of the paper, casting ashadow on to the screen, S. AB represents a wave-frontarriving at the straight edge, E, at a certain instant. Ifwe consider the illumination at P, the edge of thegeometrical shadow, we see that the edge E cuts straightacross the centre of the system of zones, because PE isperpendicular to the wave-front and therefore E is thecentre of the zones. Since each element is halved theillumination at P will be half that if the obstacle were
The science of light . Edge. — InFig. 36, O representsthe illuminated aper- a ture and E a straight FlG- 36* edge perpendicular to the plane of the paper, casting ashadow on to the screen, S. AB represents a wave-frontarriving at the straight edge, E, at a certain instant. Ifwe consider the illumination at P, the edge of thegeometrical shadow, we see that the edge E cuts straightacross the centre of the system of zones, because PE isperpendicular to the wave-front and therefore E is thecentre of the zones. Since each element is halved theillumination at P will be half that if the obstacle wereabsent, quarter that due to the first zone. For apoint Q the point C will be the centre of the zones,and there will be a complete half of the system ofzones above C. Below C we shall have in additionthe other half of a few of the more central zones. First suppose CE is just the width of the first zone,then the illumination at Q is due to half of the wholesystem of zones plus half of the first zone. The intensity. 62 THE SCIENCE OF LIGHT will therefore be three-quarters that due to the firstzone, or three times as much as at P. Now suppose CEis the width of two zones, then the illumination at Qwill be due to half of the whole system plus half of thefirst two zones. The first two zones practically neutraliseone another and therefore the illumination will be equalto that due to half of the whole system, will be thesame as at P. Moving Q still further out until CE is thewidth of three zones, two of these three neutralise oneanother, leaving the third effective, so that we shall againget the intensity equal to that due to half the system plushalf of one zone, three-quarters of that due to thefirst zone. Moving Q further and further out it passesthrough a series of maxima and minima of brightnesswhich get less and less marked and shade off into uni-form illumination. Since the size of the zone depends upon the wave-length the maxima of brightness will be in different
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectlight, bookyear1910
