Wave
lastmod 2021-02-10created 2021-01-212 edits
In physics, a wave (波動) is a phenomenon in which motion or energy is transmitted through a medium. Over time the energy spreads out through space, but the medium itself only mediates the motion and does not travel. However, a wave should be defined regardless of whether a medium exists. Electromagnetic waves are waves that propagate without a medium, and in quantum mechanics wave nature is described as a fundamental property of matter, a fundamental concept defined without a medium.
If you throw a pebble into a lake, the spot where the pebble fell shakes and ripples can be seen spreading out from it. However, the water does not move along the x-axis; only the motion along the y-axis propagates. The wave produced here is a spherical wave and a transverse wave.
A wave in which the medium moves perpendicular to the direction of the wave is called a transverse wave, and a wave that compresses and expands the medium along the direction of propagation is called a longitudinal wave. Sound waves are longitudinal waves; in this case the compression and rarefaction of the air form in the same direction as the wave. As with water waves, which are transverse waves, there is no movement of the medium (matter) along the x-axis; the only difference is that the motion (displacement) of the medium is also along the x-axis.