The science of light . (i), and theangle between the refracted wave-front and the surfacethe angle of refraction (r). 1T? ., ., . BC , . AD 1 Evidently sm *=-r-p and sin /•=-£-—. Therefore — =^rr;=T> sm r AD v1 sin iso that whatever the size of the angle i, —— is always sin t the same. This is one of the laws of refraction derivedexperimentally by Snell in 1621, and the fraction sm r -=? is called the refractive index of the glass withv1 ° respect to air. It is evident that when the wave passes from a medium in which its speed is greater into a 1 See note at the end of the chapter. 28
The science of light . (i), and theangle between the refracted wave-front and the surfacethe angle of refraction (r). 1T? ., ., . BC , . AD 1 Evidently sm *=-r-p and sin /•=-£-—. Therefore — =^rr;=T> sm r AD v1 sin iso that whatever the size of the angle i, —— is always sin t the same. This is one of the laws of refraction derivedexperimentally by Snell in 1621, and the fraction sm r -=? is called the refractive index of the glass withv1 ° respect to air. It is evident that when the wave passes from a medium in which its speed is greater into a 1 See note at the end of the chapter. 28 THE SCIENCE OF LIGHT medium in which its speed is less the refractive indexis greater than unity, and vice versa. When thewave-front is parallel to the surface each point willreach the surface at the same time and therefore eachpoint will start sending out a secondary wavelet intothe glass at the same time. The refracted wave-frontwill therefore also be parallel to the surface and willgo on Fig. by Parallel Plate.—If a plane wave passesthrough a plate of the medium with parallel surfaces itis evident that at the second surface it will be deviatedthrough the same angle into the original direction, thewave being simply displaced sideways by its passagethrough the plate. AB, CD, EF, and GH, in Fig. 13,represent successive positions of the wave-front passingthrough the plate, which is shaded. The amount ofsideways displacement depends upon the obliquity ofthe wave to the surface as well as on the thickness ofthe plate. The displacement is evidently zero when the REFLECTION AND REFRACTION 29 wave is parallel to the surface and is greatest when theobliquity is greatest, when the wave-front is almostperpendicular to the surface. Refraction by a Prism.—If the second surface of the > „.
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectlight, bookyear1910
