Simplest form of lens
A glass sphere is the geometrically simplest form of lens. In this photo, one can clearly see how the lens magnifies the lines in the background. The spherical aberrations towards the edges are clearly visible, as well as another fascinating effect: the straight lines gradually become distorted into closed curves when looking towards the edges perpendicular to the lines in the background. Spherical lenses smaller than a millimeter in diameter are often used at the tip of an optical fiber for coupling light in and out of the fiber.
—Susanne Viezens, Max Planck Institute for the Science of Light, Erlangen, Germany