2.5 Motion of wave during reflection

  Superposition of Waves

• A pulse traveling along the string is reflected when it reaches the end.
• If the end is fixed, the reflected pulse becomes inverted. This is because when the string reaches the wall, the string pulls up the wall. According to Newton's third law, the wall also exerts an equal and opposite force. Due to this force, the pulse becomes inverted.
• If the end is free, the reflected pulse remains same but propagates in opposite direction.

• The formation of the reflected pulse is similar to the overlap of two pulses traveling in opposite direction. The net displacement at any point is given by the principle of superposition.

• When the pulse reaches the boundary between a light string and a heavy string, it undergoes partial reflection and partial transmission.
• The tension in both the string is same. So, the velocity of the wave depends on the mass density of the string.
• When the pulse approaches from the light string, the heavy string behaves like a fixed end. So, the reflected pulse becomes inverted. The heavy string can also move. So, the part of the original pulse is transmitted to the heavy string.

• When the pulse approaches from heavy string, the light string offers little resistance and behaves as free end. So, the reflected wave is not inverted.