Complex sentence: A Luneburg lens (originally "Lüneburg lens", often incorrectly spelled "Luneberg lens") is a spherically symmetric, variable-index refracting structure which will form perfect geometrical images of two given concentric spheres onto each other.
Split into simpler sentences: A Luneburg lens (originally "Lüneburg lens", often incorrectly spelled "Luneberg lens") is a spherically symmetric lens that has varying index of refraction inside it. For certain index profiles, the lens will form perfect geometrical images of two given concentric spheres onto each other.