• Compression wave: A compression wave is a series of compressed and less compressed molecules in a medium (like air). In the context of sound waves in a pipe, visualize the incident wave as a group of compressed molecules moving towards the open end.

• Atmospheric Pressure: Because the pipe is open, the pressure at the open end must always be atmospheric, which is the pressure exerted by the Earth's atmosphere at any given point.

• Reflection: When the compression wave reaches the open end, it's no longer restrained by the pipe walls. The wave spreads out until its pressure drops to atmospheric pressure. This leads to a rarefaction (region of below-average pressure) that propagates down the pipe away from the open end. This results in a pulse of high pressure travelling towards the open end, reflected as a pulse of low pressure travelling away from the open end.

• π Phase Change: A π phase change in the pressure wave occurs during the reflection process.

• Standing Waves: At an open end, the standing wave has a (displacement) antinode, where the molecules are free to move. At a closed end, the standing wave has a (displacement) node, where the wall prevents molecules from moving along the pipe.

Think of a crowd at a concert, with people pushing forward to get closer to the stage. If the crowd suddenly finds a wide-open space (like an open end of a pipe), they would spread out quickly, accelerating until they're moving at their highest speed. This is similar to a compression wave reaching the open end of a pipe.