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270 Polarisation toy double refraction. A double refract- ing crystal supplies the means of obtaining polarised rays by the first method. When a ray of common light is incident upon such a crystal in a plane passing through its axis, it will be divided, as has been already explained, into two rays, the ordinary and extraordinary, both of which will be found to be polarised if examined by the test already explained. The plane of polarisation of the ordinary ray will coincide with the plane of the angle of incidence, and the plane of polarisation of the extraordinary ray will be at right angles to it. Thus the double refracting crystal resolves the ray of common light into its two component polarised rays, exactly as a common prism resolves a ray of solar light into its component rays of dif- ferent refrangibility. prev next |