The science of light . m. Y is another lead foil screen similar to X, butwith two pinholes, A and B. These must be very smalland very close together, their distance apart being lessthan half a millimetre. The easiest way to pierce themis to place the foil on a sheet of glass and pierce with afine needle. Z is a white screen to receive the light 48 THE SCIENCE OF LIGHT coming from A and B, but it is easier to use a magnifying-glass or eye-piece, E, fixed on a little stand instead ofthe screen Z. It is convenient to have the three screensabout a foot apart. The waves of light emerging from P fal
The science of light . m. Y is another lead foil screen similar to X, butwith two pinholes, A and B. These must be very smalland very close together, their distance apart being lessthan half a millimetre. The easiest way to pierce themis to place the foil on a sheet of glass and pierce with afine needle. Z is a white screen to receive the light 48 THE SCIENCE OF LIGHT coming from A and B, but it is easier to use a magnifying-glass or eye-piece, E, fixed on a little stand instead ofthe screen Z. It is convenient to have the three screensabout a foot apart. The waves of light emerging from P fall on the secondscreen, and two small fractions of them emerge throughA and B and act like the two sources Sx and S2, in Fig. 28,producing interference bands at the screen Z or theeye-piece E. The bright and dark bands are ratherfaint if received on a screen, but are very clear whenviewed through an eye-piece. Monochromatic Sources of Light.—Interference bands,or fringes as they are frequently called, are very much p). Fig. and more numerous when a source of light isused which only gives out waves of a certain definitewave-length. Such sources are produced when certainmetallic salts are placed in the colourless bunsen flame,the most convenient salts for general use being the saltsof sodium such as common salt or carbonate of sodium flame, which is a bright yellow colour, reallygives out waves of two different lengths, but they areso very nearly equal that for a great many purposesthey act as waves of a single length. There are some much easier ways of producing inter-ference bands than by Youngs method, and thereforea few of these methods will be described briefly. Lloyds Single Mirror Method.—In this method anarrow slit S, Fig. 30, illuminated by a sodium flame, is INTERFERENCE 49 placed just at the end of a sheet of plate-glass 30 or 40centimetres long and 3 or 4 centimetres broad. It isparallel to the surface of the glass and a very smalldistance above
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectlight, bookyear1910
