Purpose: To demonstrate that seismic waves reflect when they encounter a change in rigidity or incompressibility.
Supplies: Slinky
Background and Demonstration:
Following the previous demonstration, we know that compressional and shear waves travel down the slinky, and their speeds are related to the slinky's incompressibility and rigidity. When these waves encounter a material with a different incompressibility or rigidity, some or all of the wave will be reflected. To demonstrate this, ask a helper to hold one end of a stretched-out slinky, and to try to hold the end as stiffly as possible. Your helper now represents a material with a much greater rigidity than the slinky. Send an S wave down the slinky (by moving your end in a quick up-down fashion) and note that a wave reflects from your helper and propagates back to you. [It will also reflect from you and continue back again]. Now ask the students to pay attention to the initial direction of the wave (up, if you started the wave with an upward motion of your hand). The reflected wave will have the opposite initial direction. That is, an S wave reflecting from a more rigid material will reflect with the opposite polarity of the incident wave.
To demonstrate reflection from a less rigid material is much more difficult. [I am often unsuccessful, but the students get the point anyway.] One approach is to suspend the slinky vertically, holding it from the top. [I usually stand on the table for this one; that grabs their attention.] The bottom end is free, so the material below the slinky (the air) has much lower rigidity than the spring. If you can send a single S-wave pulse down the slinky, the free end should move with about twice the amplitude of the incident pulse, generating a reflected wave with the same polarity as the initial pulse (if your pulse starts to the right, the first part of the reflected wave should also move to the right). Unfortunately, it is very difficult to send a single pulse down a suspended slinky with a free end.
For smaller groups, it may be possible to do both of these demonstrations with the slinky stretched on the floor. The floor damps out spurious oscillations, making the pulse of interest clearer; of course it also damps out this pulse, so you have to start with a fairly large pulse. To simulate a smaller rigidity in this case, tie a long length of fishing line to one end of the slinky. The fishing line is only slightly less rigid than the slinky, so some of the wave will continue down the string. If the string is long enough, the reflection from the far end of the string should arrive late enough that it does not disrupt the waves on the slinky (hopefully). At any rate, the students should pay attention only to the waves in the slinky.
It is also possible to carry out these demonstrations using compressional or P waves. However, it is often difficult to distinguish initial compressions from initial dilatations. Also, since S waves in a slinky are much easier see at a distance, P-wave demonstrations are probably appropriate only for smaller groups.
Jeffrey S. Barker (SUNY Binghamton) Demonstrations of Geophysical Principles Applicable to the Properties and Processes of the Earth's Interior, NE Section GSA Meeting, Binghamton, NY, March 28-30, 1994.
Questions or comments: jbarker@binghamton.edu
Last modified: March 18, 1996 (content), June 6, 2021 (reformatted and moved to Google sites)