Michelson A. A. Light waves and their uses (1903)

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32 Light Waves and Their Uses

nation we*re so powerful as to endanger the specimen, or to introduce* temperature variations which would vitiate the results of the measurement.

It is apparent from all that precedes that in all measurements by the microscope or the telescope we are, in fact,

FIG. 26

making use of the interference of light waves. Let us see, then, if we are making the best use of this interference*, or whether it may not 1 Mi possible to increase the high degree of accuracy already attained.

It has just, been shown that, in the case of a telescope, tin* angular magnitude* of the diffraction rings, and with this the* accuracy of me»asure*ment. e>f the* position of the luminous point, depends only on the* diameteT of the* objective*. Now. the form of the* fringe*s will of course vary with the* form of the aperture*, and if this be* sepiare* instead of circular, the* diffraction image* will be* represente*d bv Fig. 2(*>, which may be compared with Fig. 23. The* width of the* fringes is but littl«‘ altered, while* then* is a perceptible increase in dis

Microscope, Telescope, Interferometer 38

tinctness. Let the middle part of the aperture now be covered up, as in Fig. 27, so that the light can pass through the uncovered portions, a and 6, only.

Fig. 28 shows the appearance of the fringes in this case. The distribution is somewhat different, but the distinctness is considerably increased, so that the position of the center of any fringe (the central bright fringe, for instance) may be measured with a decided increase in accuracy. The utilization of the two portions of a lens? at opposite ends of a diameter, converts the telescope or microscope into an interferometer.

This term is used to denote any arrangement which separates a beam of light into two parts and allows them to reunite under conditions to produce interference. The path of the separated pencils may be varied in every possible way;

for instance, by interposing prisms or mirrors, provided the optical paths are nearly equal and the angle between the two final directions very small. The first condition is essential only when the light is not homogeneous. The reason will be apparent when it is remembered that the width of the interference bands depends on the wave length of the light employed. If the light is composite, as in the case of white light, each component will form interference bands whose width is proportional to the wave length.

This is illustrated in Fig. 29, where the fringes due to red, yellow, and blue light respectively are separated. In

FIG. 28

FIG. 27