normally incident on two narrow parallel slits which are separated by a small distance 
[P6.1]
is: a classic demonstration of interference. [P6.1]
has as input: light of wavelength normally incident on two narrow parallel slits which are separated by a small distance [P6.1]
has as output: an interference pattern consisting of alternate bright and dark bands (interference fringes) on a distant screen. The bright fringes of order 
are observed at angles 
from the straight-through direction given by
where 
[P6.1]
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