Fresnel Mirrors

(also Fresnel bimirror), two plane mirrors that form a dihedral angle a few angular minutes less than 180° used for the observation of the phenomenon of the interference of coherent light beams; proposed by A. J. Fresnel in 1816.

Figure 1. Diagram of Fresnel’s experiment

When the mirrors I and II (see Figure 1) are illuminated by a source S, the beams of rays reflected from them may be considered to have originated from coherent sources S₁, and S₂, which are virtual images of S. Interference occurs in the region where the beams cross. If S is linear (a slit) and parallel to the edge of the Fresnel mirrors, then upon illumination by monochromatic light the interference picture in the form of parallel slits of evenly spaced dark and light bands or fringes is observed on the screen M, which can be set up anywhere in the region where the beams cross. The wavelength of the light can be determined from the distance between the bands. Experiments conducted with Fresnel mirrors were one of the crucial proofs of the wave nature of light.