Interference rings by scattered light

An experiment on light interference, which can be carried out at a very low cost and more easily than in the past, is revisited. This simple experiment is made possible by the availability of inexpensive collimated laser pointers. The observed circular interference pattern is beautiful; it is a source of wonder; and its interpretation is straightforward. Unlike many interferometers, the one presented in this work does not require alignment, and the interference fringes are well contrasted, stable, and clearly visible even to a large audience in a non-darkened environment. The explanation of the phenomenon has been developed in a straightforward manner, closely aligned with the experimental conditions. In addition, an original implementation is shown that allows the determination of the refractive index or thickness of thin sheets of transparent materials. Finally, a conceptually new interferometric device for determining small displacements and their variations is presented.