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
34
Light Waves and Their Uses
the actual experiment, however, they are all superposed. At the middle point, where the two paths are equal, all the
colors will be superposed, the re-
Jsult being a white central band. 1 At no other point will this be true, I III II an j resuit wiu be a series of
colored fringes symmetrically disposed about the central white fringe, the succession of colors being exactly the same as in the case of thin films (c/. Plate II).
The breadth of the fringes is determined by the smallness of the angle under which the two pencils meet. This is shown in Fig. 30. FIG gg In the right-hand figure the angle
between the pencils is smaller than in the other, while the breadth of the fringes is correspond-ingly greater in the former than in the latter. The exact
FIG. 30
relation is readily obtained. We have only to note that ac is the wave length I (very nearly) and be is (very nearly) the width b of a fringe; whence, if e is the very minute angle