File:X rays and crystal structure (1915) (14778024195).jpg

Identifier: xrayscrystalstru00braguoft (find matches)
Title: X rays and crystal structure
Year: 1915 (1910s)
Authors: Bragg, William Henry, Sir, 1862-1942 Bragg, William Lawrence, Sir, 1890-1971
Subjects: Crystallography X-rays
Publisher: London : G. Bell
Contributing Library: Gerstein - University of Toronto
Digitizing Sponsor: MSN

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ogeneous ray does not take placeexactly on a vertical line on the crystal face, but at anumber of points scattered over the face and groupedmost densely near the vertical line. If the rays werevisible, an eye placed at the ionisation chamber slitwould see an effect like that of light reflected byslightly rippling water. To make the comparisoncomplete, it would be necessary to imagine the lightto be capable of reflection at one angle only : sothat the eye would only see this effect at its best inone position, and if it moved would soon cease to seeany reflections at all. Let PQ in Fig. 9 represent the crystal face re-flecting the ray OP of a certain wave length alongPD, as in Fig. 8. Suppose that at Q there is asmall crystal element set at a slight angle to the face THE X-RAY SPECTROMETER 33 PQ, and also reflecting the same ray. Let us findto what point it reflects it. Join OQ, meeting- thecircle through OPD in R. We know that the rayOR would be reflected by the crystal if its face was
Text Appearing After Image:
Fig. 9. in the position PRT. Consequently the face of thereflecting element at Q must be parallel to PR, andthe reflected ray QD must be parallel to RD. Let OP = Z, PQ = I. Draw QN perpendicular toRD. The triancrles OOP and POR are similar,for they have a common angle at O, and theangles PRQ and OPQ are equal to each other,being each equal to the glancing angle of reflectionfor the ray in question, which may be put equal to 0. Hence QR = PIL, and QN^PsmiOlL. This quantity is usually quite small. If, for example,/ was as much as i cm., L equal to 12 cm. and2O to 20°, QN=.^^2J12 = .028 cm. The ray reflected at R would pass by /^ at adistance of a third of a millimetre only, and the slitat D need only be opened to that extent to takein such rays. When, therefore, a crystal is not very regular in B.R. c 34 THE X-RAY SPECTROMETER its general construction, the intensity of the radiationentering- the sHt at D is much increased by wideningthe incident pencil ; that is to say, by opening thes

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