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In a Young's double-slit experiment a monochromatic beam is incident on the slits, and a thin glass sheet is then placed in front of the lower slit S2. After inserting the sheet, where can the central (zero-order) bright fringe be located on the screen?

1. at O only
2. at O or below O only
3. at O or above O only
4. Anywhere on the screen

Correct answer: at O or above O only

Solution

Placing a glass sheet in front of S2 (the lower slit) increases the optical path of the lower beam. The zero-path-difference point (central fringe) shifts toward the side of the slit that carries the slab, i.e. toward S2 (downward source) which moves the fringe upward toward S1 side... more precisely the central maximum shifts toward the slit covered by the slab. With S2 the lower slit covered, the pattern shifts upward (toward O and above O). Hence the central fringe is at O (if no slab effect) or above O once the slab is present.

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