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"An optical interferometer is a device in which two or more light waves
are combined together to produce interference. I assume the question is about an
optical interferometer combining light from several telescopes. If two
telescopes observe the same star and the light beams are superimposed, a
phenomenon called interference occurs. The star's brightness will vary with the
delay between the arrival times of the two light beams. If the arrival times are
the same, the crest of one wave will add up to the crest of the other wave
producing a new wave with twice the amplitude and four times the energy of a
single wave. Yet as the earth rotates, one telescope becomes closer to the star
than the other. When it is closer by half of a wavelength, a crest in one beam
will correspond to a trough in the other beam, and the two light waves will
exactly cancel each other, making the star disappear.
"The star reappears and disappears every time the delay between the
telescopes is a multiple of the wave period. In other words, the star looks as
if it were moving behind a dark picket fence. The 'pickets' are called
interference fringes. The wider apart the telescopes are, the smaller the fringe
spacing (the tighter the picket fence). Two telescopes only a few meters apart
produce fringes as close as the diameter of a giant star like Betelgeuse. In
this case, the star is larger than the size of a picket. It can no longer hide
behind it, that is, it no longer disappears.
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