12.4. Reflection of Mechanical Waves http://www.ck12.org
12.4 Reflection of Mechanical Waves
- State the law of reflection.
- Solve problems using the law of reflection.
- Given data about the media on either side of a barrier, determine whether the reflected wave will be upright or
inverted.
When mechanical waves strike a barrier, at least part of the energy of the waves will be reflected back into the
media from which they came. You experience this every single day, when you look in the mirror and see your own
reflection.
Reflection of Mechanical Waves
When a wave strikes an obstacle or comes to the end of the medium it is traveling in, some portion of the wave is
reflected back into the original medium. It reflects back at an equal angle that it came in. These angles are called
theangle of incidenceand theangle of reflection. The normal line, the incident and reflected rays, and the angles
of incidence and reflection are all shown in the diagram sketched above. Thelaw of reflectionstates that the angle
of incidence equals the angle of reflection. These rules ofreflectionapply in the cases of water waves bouncing off
the side of a pool, sound waves echoing off a distant cliff, or wave pulses traveling down a rope or a slinky.
Consider the change that would occur with a light rope joined to a heavier rope. When a wave pulse travels down
the rope and encounters the media change, a reflection will occur. Look at the image below. In the top sketch, we
see a lightweight (black) rope attached to a heavier rope (red). There is a wave pulse traveling down the rope from
left to right. When the wave pulse encounters the barrier (the change in rope weight), part of the wave moves into
the new medium and part of the wave is reflected back into the old medium.
As you can see in the bottom half of the diagram, the transmitted portion of the wave continues into the new medium
right side up. The transmitted wave is somewhat diminished because some of the energy of the wave was reflected
and also because the rope to be lifted is heavier. The reflected wave is also diminished because some of the energy
was transmitted through the barrier. The reflected wave is also inverted (upside down). This is a general rule for
mechanical waves passing from a less dense medium into a more dense medium, that is, the reflected wave will be
inverted.