When interacting they also merge. Otherwise they remain in their own states. Forms are basically different in material and appearance. Sounds are fundamentally different in pleasant or harsh quality. Darkness is a word for merging upper and lower.
Light is an expression for distinguishing pure and defiled.
The four gross elements return to their own natures like a baby taking to its mother. Fire heats, wind moves, water wets, earth is solid. Eye and form, ear and sound, nose and smell, tongue and taste. Thus in all things the leaves spread from the root. The whole process must return to the source.