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A cylindrical Fresnel lens, mounted horizontally on a rotating pedestal, casts a single, narrow beam of light onto the floor, walls, and ceiling of a room. As the lens rotates around the horizontal axis, the vertically oriented band of light sweeps slowly across the surfaces of the room. The viewer perceives the light only where it strikes one of the room’s surfaces, giving the impression that the band of light is emitted from the lens not as a disc but in the shape of the room’s cross section – rectangular or otherwise.