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I performed Morley experiment using He-Ne laser. Two glass slabs were there and circular fringes were formed but when I removed one slab elliptical fringes were there (and less intense fringes). Why?
In general is there any benefit of using two slabs?

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    $\begingroup$ The general answer to question like this concerning interferometers is "follow the optical path down various branches; you want to make those paths as similar as possible". $\endgroup$ – dmckee Oct 27 '13 at 16:32
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Look carefully at the two paths that the split beam of light covers, and think about the glass that the light goes through.

Beam #1 bounces off the front of the beam splitter, goes over and off the front of a mirror, comes back and goes through the beam splitter at a $45^{\circ}$ angle. (One pass through the glass of the beam splitter)

Beam #2 goes through the beam splitter, over to the other mirror, comes back and goes through the beam splitter from behind at a $45^{\circ}$ angle, bounces off the back of the half-silvered surface, and goes back through the beam splitter again at a $45^{\circ}$ angle. (Three passes through the beam splitter glass)

Adding a clear piece of the beam splitter glass at a $45^{\circ}$ angle to the first path makes the two beams optically equivalent.

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