Question: In closed organ pipe, the sound gets reflected from the rigid boundary. This makes sense. But, in open organ pipe there is no boundary at the end. So, how can sound reflect?
Answer: One is accustomed to seeing objects getting bounced from rigid wall and not from empty space. Hence, this may look odd if something is said to be reflected from the open end. Sound is not an object but is disturbance. The sound wave is reflected whenever medium changes or there is discontinuity in the medium. In open organ pipe when the sound reaches the open end, there is a change from a narrow region to a vast region. Because of this discontinuity in the medium, reflection takes place.