Fresnel's Rhomb
   Fresnel's Rhomb was developed in 1817 by Augustin Jean Fresnel (1788-1827) to produce circularly polarized light. In this form, there are two electric field vectors associated with the ray of light that are out of phase with each other by 90°. 

   The rhomb has outside angles of 54° and 126°. The light coming into the top of the rhomb was polarized at an angle of 45° to the plane ABCD. Upon reflection at R at Brewster's angle, one of the components of the linearly polarized light is retarded by 45°, and the same thing occurs at F. Thus, one of the components of the light is retarded by 90°, the condition for circularly polarized light.

   The double rhomb at the left. below is at Glasgow University and was made by Soliel of Paris before 1850. The one at the right is at the United States Military Academy.

   This rather beautiful example of Fresnel's Rhomb is unmarked. It is on display at the Magic Lantern Museum in San Antonio, Texas, which represents many years of dedicated work by its curator and owner, Jack Judson.

   This is a single rhomb, and the only one I have ever seen.

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