Laser projected through Fabry-Perot interferometer shows pattern.

In this setup, a 30 mw He-Ne laser beam is sent through a x10 microscope objective, projecting light onto one surface of the Fabry-Perot interferometer. Multiple reflections occur in the interferometer cavity, and the resulting circular fringes are displayed on a screen about twenty feet away. The Fabry-Perot interferometer consists of two parallel clear glass plates that are partly silvered on the facing interior sides, about a cm. apart. One plate is fixed, and the other plate can be adjusted in or out slightly with adjusting screws. This device uses the Haidinger fringes produced in the transmitted light after multiple reflections in the air cavity between the silvered glass planes. Looking at Fig.1 above, Ray1 of monochromatic light (from a He-Ne laser) passes through partly silvered glass plate A, then bounces off partly silvered plate B, and so on. Ray2 is parallel to Ray1 and interferes with the reflection of Ray1. The condition for reinforcement giving a maxima is 2d/Cos_ = n_. For _ = 0 deg , 2d = n_. The resulting fringe patterns projected onto a screen are a set of concentric rings similar to, but much sharper than, those of a Michelson Interferometer. Note: The laser light, after passing through the microscope objective is a narrow cone, and thus not completely parallel when entering the interferometer. This does, in theory, make the circular fringes somewhat fuzzier, but it is imperceptible. Expanded laser beam through Fabry-Perot Interferometer produces rings. Ref.: Fundamentals of Physical Optics by Jenkins & White, 1st Ed., p.93