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*Question:* Coherent light of wavelength from a
He-Ne laser falls on a double slit with a slit separation of
.
An interference pattern is produced on a screen from the slits.
Calculate the separation on the screen of the two fourth-order bright
fringes on either side of the central image.

*Solution:* The easiest way to handle this problem is to calculate
the distance of the fourth-order bright fringe on one side from the
central image, and then double this value to obtain the distance
between the two fourth-order images. From Eq. (375),

The distance between the two fourth-order fringes is therefore

Richard Fitzpatrick
2007-07-14