As a wave travels through a medium, it will ripple tank animation reach the end of the medium and encounter an obstacle or perhaps another medium through which it could travel. One example of this has already been mentioned in Lesson 2. A sound wave is known to reflect off canyon walls and other obstacles to produce an echo.
Fixed End Reflection First consider an elastic rope stretched from end to end. One end will be securely attached to a pole on a lab bench while the other end will be held in the hand in order to introduce pulses into the medium. If a pulse is introduced at the left end of the rope, it will travel through the rope towards the right end of the medium. A portion of the energy carried by the pulse is reflected and returns towards the left end of the rope. The disturbance that returns to the left after bouncing off the pole is known as the reflected pulse. A portion of the energy carried by the pulse is transmitted to the pole, causing the pole to vibrate. Because the vibrations of the pole are not visibly obvious, the energy transmitted to it is not typically discussed.
The focus of the discussion will be on the reflected pulse. What characteristics and properties could describe its motion? When one observes the reflected pulse off the fixed end, there are several notable observations. First the reflected pulse is inverted. That is, if an upward displaced pulse is incident towards a fixed end boundary, it will reflect and return as a downward displaced pulse. Similarly, if a downward displaced pulse is incident towards a fixed end boundary, it will reflect and return as an upward displaced pulse. The inversion of the reflected pulse can be explained by returning to our conceptions of the nature of a mechanical wave.
The speed of the reflected pulse is the same as the speed of the incident pulse. The wavelength of the reflected pulse is the same as the wavelength of the incident pulse. The amplitude of the reflected pulse is less than the amplitude of the incident pulse. Of course, it is not surprising that the speed of the incident and reflected pulse are identical since the two pulses are traveling in the same medium. This sequence photography photo shows an upward displaced pulse traveling from the left end of a wave machine towards the right end.
The wave reflects off this fixed end and returns as a downward displaced pulse. Reflection off a fixed end results in inversion. Free End Reflection Now consider what would happen if the end of the rope were free to move. Instead of being securely attached to a lab pole, suppose it is attached to a ring that is loosely fit around the pole. Once more if a pulse is introduced at the left end of the rope, it will travel through the rope towards the right end of the medium. When the incident pulse reaches the end of the medium, the last particle of the rope can no longer interact with the first particle of the pole. Since the rope and pole are no longer attached and interconnected, they will slide past each other.