The scalar wave can be physically observed in a standing wave pattern. A
wave form is contained within an area by a barrier. As the wave hits the
barrier it returns back in the direction it originally came from. The
wave form of the reflected wave will interfere with the original wave.
For example, two crests may cross one another, in which case a new crest
will appear which is much larger, a product of the energy of the two
crests combined. Or a crest traveling in one direction may cross a
trough traveling in the other direction; in this case the two patterns
appear to cancel each other out.
Usually, the crossing wave patterns will not coincide exactly trough to
trough, crest to crest. Various complicated interference patterns appear
and the wave forms cross and re-cross the medium.
However, sometimes the frequency of vibration, wave length, and the size
of the area which contains the wave combine in such a way that the
crossing wave forms coincide exactly with each other, trough to crest,
180 degrees out of phase. We then see a “standing wave pattern,” so
called because certain points along the medium seem to be standing
The points which seem to be standing still, points of no displacement,
are called nodes. Other points along the medium seem to vibrate up and
down, without moving longitudinally, from positive displacement to
negative displacement (trough to crest). These points of maximum
displacement are called antinodes. You can see a GIF animation of this
phenomenon at the
Multimedia Physics Studios.