One of my favorite areas of instrumental music was developed by two of its most well-known ambassadors of the era: Debussy and Ravel. It was a brief area but it proved a major turning point for music which transitioned from mainstream classical to jazz and a wide range of other music.
The beginning of so-called Impressionism is a blurred line. We always think of Debussy who had connections and drew inspiration from artists such as Monet, Pissarro and Renoir. The idea behind the music was to move away from music that was emotionally charged and creating an image of a landscape. Although being inspired by Impressionism, Debussy did not like being referred to as an impressionist composer. A lot of his musical influences came from Gounod and Massenet, plus the Russian composers such as Tchaikovsky, Balakirev (who wrote Islamey) and Rimsky-Korsakov.
The latter composer wrote Scheherazade (also influencing Ravel) which was full of orchestral tone colours and using different modes based on the Arabic scale. Debussy borrowed the use of pentatonic scales with an Oriental influence of Javanese music, Whole tone scales for a dreamy setting and older modes for an Ancient setting. The combination of these different sounds appeal to the imagination in many ways. Creating a story from different places.
Another key element for the piano especially was creating tone colors with pedaling effects to create watery like sounds. A particular example is Images. In Ravel's case- Jeux d’eau, Ondine, and Une Barque Sur I’Ocean are prime examples.
Debussy and Ravel had created a new sense of harmonic writing which filtered into jazz harmony. Ravels technically was very much linked to Liszt. Debussy in terms of musicality closer to Chopin. Previous composers to them such as Chabrier had already paved a way into the new way of writing. Saint Saens was completely against the idea, yet ironically had written Carnaval of the Animals. In the case of Aquarium, it can be argued as being programmatic and similar to Debussy's idea of this watery ‘impression.’
Debussy was rebellious against the composing tradition but had won the Prix de Rome composing prize at the time. Ravel, in particular, had continued the new found style of music we know as Neoclassicism. As well as creating music that had powerful imagery, he wrote music that paid homage to the French Baroque composers such as Rameau and Couperin. Creating music from the Baroque tradition with a modern twist. The direction of music had reached the Americas as Gershwin was a student of Ravel. A lot of ideas were exchanged with the new harmonies and orchestration. Gershwin wrote An American in Paris while Ravel added a lot of jazz rhythm in his Piano Concerto in G and influences from Blues music became apparent.
As a side note: the classical composer Dvorak also traveled America with students there before the early development of Blues music came about. Meanwhile, in Europe, this ‘impressionistic’ influence became widespread. In England, composers such as John Ireland wrote piano music based on this style. Vaughan Williams wrote The Lark Ascending for violin and orchestra. Eminent violinist Eugene Ysaye from Belgium created a very similar manner to Debussy and Ravel but for violin. Even today where Debussy took a lot of influences from the East; the music from the popular Anime movies from Japan (note the use of the French word) took a lot of inspiration from Debussy and Ravel's style of orchestral writing. The highly atmospheric sound of the orchestra and higher ranges of the strings paint a vivid colorful picture. The influence took a full circle return to the background of these very original movies.
Whenever you have time, listen out for the soundtrack of these - Merry Christmas Mr. Lawrence (not an Anime per se but same music style. Sounds like Debussy’s Suite Bergamasque) and Ravel's Feria, Daphnes et Chloe and Ma mère l'Oye for some incredible examples.