Answer: Reflection
Step-by-step explanation:
The incident wave is the wave coming from the source. When an incident wave hits an interface, it splits into two parts. There is the reflected wave and the transmitted wave. Assuming that the incident wave hits the interface directly, and not at an angle, the reflected wave will reflect back to the source, and the transmitted wave will pass through the interface and continue on its path. The reflected wave will be "upside down" as it gets sent back.