"Ultraviolet"/"The Ballad of Paul K" is the fourth and final single from British pop-rock band McFly's second studio album, Wonderland. It is a double A-side single and was released on 12 December 2005. The single peaked at No. 9 on the UK Singles Chart, making it one of five singles in their career to date to not reach the top five, along with "Do Ya/Stay with Me" at No. 18, "Falling in Love" at No. 87, Party Girl at No. 6, and That's the Truth at No. 35.
Both tracks were written by McFly members Tom Fletcher and Danny Jones, with "The Ballad of Paul K" being written also by Dougie Poynter, who was the principal writer. The Ballad of Paul K was based on the members of Mcfly's fathers. The song is about a middle-aged man coming to terms with his life. The name "Paul K" came from a boy Dougie knew at school, but is also a reference to Josef K in Kafka's The Trial. The song "Ultraviolet" was initially called "Summer Girls". Both songs feature on McFly's second studio album Wonderland.