CONCEPT: Reflection of waves: When a wave is incident on a boundary between two media and it returns back into the initial medium, then it is called reflection of waves. The phenomenon of echo is an example of reflection by a rigid boundary. If the boundary is not completely rigid or is an interface between two different elastic media, the situation is somewhat complicated. A part of the incident wave is reflected and a part is transmitted into the second medium. The transmitted wave is called the refracted wave. The incident and refracted waves obey Snell’s law of refraction, and the incident and reflected waves obey the usual laws of reflection. Reflection by a rigid boundary: If a pulse travelling along a stretched string and is reflected by the rigid boundary. Assuming there is no absorption of energy by the boundary, the reflected wave has the same frequency and amplitude as the incident pulse but it suffers a phase change of π or 180° on reflection. This is because the boundary is rigid and the disturbance must have zero displacements at all times at the boundary. By the principle of superposition, this is possible only if the reflected and incident waves differ by a phase of π so that the resultant displacement is zero. Reflection by a non-rigid boundary: If the boundary point is not rigid but completely free to move (such as in the case of a string tied to a freely moving ring on a rod), the reflected pulse has the same phase, amplitude, and frequency (assuming no energy dissipation) as the incident pulse. The net maximum displacement at the boundary is then twice the amplitude of each pulse. An example of a non-rigid boundary is the open end of an organ pipe. EXPLANATION: If a pulse travelling along a stretched string and is reflected by the rigid boundary. Assuming there is no absorption of energy by the boundary, the reflected wave has the same frequency and amplitude as the incident pulse but it suffers a phase change of π or 180° on reflection. If the boundary point is not rigid but completely free to move (such as in the case of a string tied to a freely moving ring on a rod), the reflected pulse has the same phase, amplitude and frequency (assuming no energy dissipation) as the incident pulse. Hence, option 2 is correct.