We used the focal line of a cylindrical lens illuminated by a collimated expanded He–Ne red laser light as a filamentary light source to observe Young’s interference fringes. When the monochromatic filamentary light source is parallel to Young’s slits, the interference fringes are parallel to them and are well defined and highly contrasted. When the filamentary light source is rotated in a plane parallel to that of the slits, the visibility of the interference fringes is always extremely high, but their geometrical distribution and the distance between them depend on the rotation angle and the observation distance. A simple theoretical model is proposed, based on a cylindrical wave propagated according to the Huygens–Kirchhoff formula. The experimental results coincide remarkably well with the stated theoretical model.
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