The invention of the musical film is credited to the American inventor Lee de Forest, who developed sound-on-film technology in 1923. This allowed for the synchronization of sound and images in motion pictures, which marked the birth of the musical film genre.

Before de Forest's invention, films were silent and accompanied by live music or a pianist. With the introduction of sound, films could now include songs, dialogue, and even full-fledged musical numbers. This change revolutionized the film industry and brought a new level of entertainment to audiences.

The first musical films were short films called 'Vitaphone shorts' that were produced by Warner Bros. beginning in 1926. These films featured live bands, vocalists, and dancers, and were shown as a prelude to feature films in theaters. These early musical shorts were a hit with audiences and helped establish the musical film genre.

The first full-length musical film was 'The Jazz Singer,' which was released in 1927 and starred Al Jolson. The film was a huge success and was the first feature-length motion picture with synchronized dialogue and singing. It marked the end of the silent film era and the beginning of the 'talkie' era.

After the success of 'The Jazz Singer,' the musical film genre continued to grow and evolve. Hollywood studios began producing more musical films, and many famous actors, such as Fred Astaire, Gene Kelly, and Judy Garland, became known for their musical film roles.

In conclusion, the invention of musical film is credited to Lee de Forest's sound-on-film technology in 1923. The early musical films such as 'Vitaphone shorts' and 'The Jazz Singer' helped establish the musical film genre, which continues to be a beloved form of entertainment to this day. The musical film has been an important part of the motion picture industry, entertaining and inspiring audiences for nearly a century.