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The finite speed of light enables us to look back in time as we peer out in the universe. This enables us to study how galaxies were formed and how they evolved throughout the history of the universe. However, it is difficult to study distant galaxies in detail: Each galaxy only covers a few pixels, even in the best images from the Hubble Space Telescope, and they are so faint that they are only barely detected even with our largest telescopes. Nature has provided us with a tool to overcome these obstacles: Light rays from distant galaxies are deflected in the gravitational field of a massive object, such as a cluster of galaxies, which acts as a lens. This phenomenon is known as gravitational lensing. If the distant galaxy and the lens are almost perfectly aligned along our line of sight, the light will be focused towards us and the distant source will be magnified. This enables us to see objects and study phenomena which would otherwise be unobservable.