Plane-parallel glass plate rests on one surface lenticular lens. In the observation of Newton's rings in reflected light sodium burner (lambda = 589 nm) found that the radius of the dark rings about k = 20 (the dark central ring corresponds to k = 0) is equal to r1 = 2 mm. When the plate was placed on the other surface of the lens, the radius of the dark ring of the same order become equal to r2 = 4 mm. The focal length F of the lens if the refractive index of the glass, from which it is made, n = 1,50.

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