The behavior of a wave reflecting at a boundary on the wave machine depends on whether the boundary is free or clamped.
Watch The Video:
Older Videos:
Wave reflecting off of clamped boundary
Wave reflecting off of free boundary
Teachable Topics:
- Traveling waves
- Reflection
- Phase shift
Theory:
When a wave encounters a boundary in its medium, it reflects off that boundary. That is, the wave is sent back in the direction it came from. If the ends of the wave machine are free, then a wave crest striking the end will be reflected as a crest, and a trough will be reflected as a trough. The wave will retain its shape as it returns in the other direction. If the end of the wave machine is held in place, then the wave reflects differently. Crests will reflect as troughs and vice versa. It is said that the wave experiences a phase shift of πradians. 
Apparatus:
- large wave motion demonstrator
- clamp
Procedure:
- Set up the large wave motion demonstrator on a flat surface.
- Make a small crest and watch it travel down the rods and reflect back again as a crest.
- Send a trough down the machine and you'll see it reflect as a trough. A free boundary doesn't change the phase of a wave.
- Now add a clamp to the very last rod, and repeat the same procedure. This time the crest is reflected as a trough!
- With the clamp still in place, send a trough down the machine... a crest will reflect back. A clamped boundary changes the phase of the wave by π radians.
