Single Transverse Pulses - Reflection of a Wave at a Free End

When a wave pulse reaches the end of the wave demonstrator, it is reflected.

Discussion

A wave demonstrator consists of a set of equal-length metal rods soldered at their midpoints to a torsional wire that runs the length of the apparatus. When a rod at an end of the apparatus is moved quickly up or down, the disturbance is transmitted as a wave pulse that travels along the demonstrator. The demonstrator used in this set of movies is 0.90 m long and has long rods and a low wave speed. In later movies, a demonstrator with short rods and a greater wave speed is used.

You can use a stopwatch to measure the elapsed time (start timing at the moment the pulse is generated and leaves one end of the demonstrator and stop timing after the pulse has traveled one or more lengths of the demonstrator) and distance traveled by a wave pulse (in multiples of the length of the demonstrator, 0.90 m) to find the wave speed. With this information you can calculate the wave speed in this long-rod demonstrator (about 0.24 m/s).

When a wave pulse arrives at the end of the wave demonstrator that can move freely, it is reflected. As the pulse moves toward the end of the wave demonstrator it is on the upper side. After reflection, the reflected pulse is still on the upper side of the wave demonstrator. There is no inversion of peaks when reflection occurs at a free end. This is characteristic of waves reflected from a free end. In contrast, after reflection from a fixed end, a reflected peak is inverted - it moves away from the fixed end on the side opposite to that on which it moved toward the fixed end before reflection. This effect was seen in the movie of wave pulses on a Slinky.