Page:Elementary Text-book of Physics (Anthony, 1897).djvu/171

§133] the opposite sides of $$c$$ or $$d$$ are always moving in opposite directions. It follows that the resultant wave has no progressive motion. It is a stationary wave. Places where no motion occurs, such as $$c$$ and $$d$$, are called nodes. The space between two nodes is an internode or ventral segment. The middle of a ventral segment, where the motion is greatest, is an anti-node. It will be seen later that all sounding bodies afford examples of stationary waves.

133. Reflection of Waves.—When a wave reaches the bounding surface between two media, one of three cases may occur:

(1) The particles of the second medium may have the same facility for movement as those of the first. The condition at the boundary will then be the same as that at any point previously traversed, and the wave will proceed as though the first medium were continuous.

(2) The particles of the second medium may move with less facility than those of the first. Then the condensed portion of a wave which reaches the boundary becomes more condensed in consequence of the restricted forward movement of the bounding particles, and the rarefied portion becomes more rarefied, because those particles are also restricted in their backward motion. The condensation and rarefaction are communicated backward from particle to particle of the first medium, and constitute a reflected wave. It will be seen that when the condensed portion of the wave, in which the particles have a forward movement, reaches the boundary, the effect is a greater condensation, that is, the same effect as would be produced by imparting a backward movement to the bounding particles if no wave previously existed. In the direct rarefied portion of the wave the movement of the particles is backward, and the effect, at the boundary, of a greater rarefaction is what would be produced by a forward movement of those particles. The effect in this case is, therefore, to reverse the motion of the particles. It is called reflection with change of sign.

(3) The particles of the second medium may move more freely than those of the first. In this case, when a wave in the first