This is a theme called "The Force Theme" or "Ben Kenobi´s Theme". In the movies 4-6 it is most often used to characterize the force´s influence or Obi-Wan´s doings. Unfortunately in the newer films, it is used more and more seldom. As the force plays an important role in the movies, it has become the perhaps most famous of all themes and the theme that binds the films together musically. It has become especially famous for its use in the "Binary Sunset"-scene in Episode 4 in which Luke walks out of the homestead and watches the two suns of Tatooine setting. In the scene you mentioned, Anakin comes out of the house just as Luke will many years later, so Williams decided to use the same theme, and also the same orchestra.