What makes the underwater background music in Little Mermaid sound so magical? It's a combination of many factors, including the harmony, melody, rhythm, articulation, instrumentation, and formal structure.
One of these factors is its use of the Lydian mode, which is often associated with wonder, magic, and dreams. Part of Lydian's charm comes from the fact that, unlike major and minor, it has major chords on both I and II. It also has a tritone between 1 and 4 which has often been used to great effect (think of the Simpsons theme or "Maria" from West Side Story).
But in the case of the Little Mermaid, things are a little more complicated. The ostinato shown above can be thought of as I-II7 in Bb Lydian, but it can also be thought of as IV-V7 in F major. If you think of it as F major, then it's as if the music is wavering around the dominant without resolving. In the song "Part of Your World," this makes sense as she's spending the first part of the song talking about how unsatisfied she is. But then, when she finally puts a name to her dream – "I wanna be where the people are" – that wavering IV-V7 finally resolves to I in F major. Ah, resolution...
So the use of Lydian here is a doubly whammy. On one hand, it already comes loaded with connotations of wonder and dreams. And on the other, it serves as a dominant prolongation of the relative major, refusing to resolve until Ariel finally puts a name to her dreams.
Sam Zerin is a PhD student in musicology at New York University and a former lecturer in music theory at NYU, Brown University, and the Borough of Manhattan Community College. He also runs Social Media Music Theory (@SocialMediaMus1)