What are the wild waves saying
Sister, the whole day long;
That ever amid our playing,
I hear but their low lone song.
-Joseph Edwards Carpenter
Our world is flush with waves. Light, sound, infrared, radio, T.V., cell, Wi-Fi, satellite, radars, microwaves, garage door openers, dog fences… if we could see them all we’d be blinded in an electronic fog. Fortunately, all those waves aren’t visible and lack the physical power to knock us around, instead of passing harmlessly by or through us, with the one unfortunate exception being the sort of waves encountered at sea.
The word “wave” has a host of entomological derivations dating back eons, all stemming from words synonymous with movement and undulations. Long before we knew of electronics and such, about the only waves spoken of were those seen upon the water. We know now that the very concept of waves is one of the most prominent in our physical universe and that the vast majority of physics deals with or is rooted in the study, understanding and utilization of waves. In the purest sense, waves are nothing more than some transient energy or disturbance passing through a medium. But what physical force they inject when traveling through the medium of water can raise a mighty crest to founder your vessel or pound a beach clean of its sand.
One of the most instructive and easy to understand types of waves and interestingly, most analogous to a wave upon the water is a relatively recent discovery: the stadium wave. The crowd stunt in stadiums (you know, when fans jump up out of their seats, arms in the air, in a sort of progressive domino effect) is said to have first been done Oct. 15, 1981 at the New York Yankees/Oakland A’s American League championship series game in the Oakland Coliseum. The University of Washington took note and made it mainstay, then the entirety of the sporting world.
The stadium wave is one basically comprised of energy in the form of the enthusiasm you can see as it travels horizontally around the stadium, even though the people only move vertically. And that’s basically the same way a wave in the water works. Although we see the wave as a solid moving thing, the water particles really only move up and down as the wave’s energy moves through the medium of water. When a wave rolls into the side of your boat, it’s the energy of the wave giving motive force to the water that causes it to rock you. Another example would be to stretch a rope taut between you and another person, and then one of you “whips” your end. The undulating vertical movements of the wave you’ve created in the rope are visible, even though the rope does not move horizontally.
Back to the afore mentioned medium. A medium is a substance or material which carries the wave… sort of. You’ve heard the phrase ‘news media’ or ‘news medium’? The news media refers to the various institutions (periodicals, television stations, radio stations, etc.) which carry the news from one location to another. The news moves through the media; the media doesn’t make the news and the media isn’t the same as the news. The news media is merely the thing that carries or transports the news from its source to various locations. In a similar manner, a wave medium is the substance which carries a wave’s energy (or disturbance) from one location to another. Again, the wave medium is not the wave and it doesn’t make the wave; it merely carries or transports the energy from its source to other locations. In the case of the stadium wave, the fans in the stands are the medium. In the case of the waves you see on the water, obviously, the water is the medium.
To fully understand the nature of a wave, it is important to consider the medium as a series of interconnected or interacting particles. In other words, the medium is composed of parts which are capable of interacting with each other. The interaction of one particle of the medium with the next adjacent particle allows the disturbance/energy to travel through the medium. Now, look at both the water and the rope as a mass of particles which make up a whole. To introduce a wave into rope or water, the first particle is displaced or moved from its equilibrium or rest position by some form of energy or movement. The particle might be moved upwards or downwards, or side to side by your wrist or a stiff breeze, but once moved it is returned to its original equilibrium or rest position, usually by gravitational forces, and thus, when done in succession from one particle to another, the energy progresses and the wave is formed.
There is always a force, very often as mentioned above gravity, acting upon the particles which restore them to their original position once the energy of the wave has passed. In a water wave, each molecule of the water always returns to its original position. And in a stadium wave, each fan in the bleachers always returns to their seat.
In the rope wave, the power which generates the wave comes from the movement of your arm. But where does the energy that makes a water wave come from? Most typically the energy of the friction and drag of the wind across the surface sets the water into motion. But in the simplest case, say the concentric outwardly emanating waves of a pebble tossed into a puddle, it’s the energy from the mass and momentum of the pebble that causes the waves. Other forms of water wave generation can be seismic bottom displacements such as those that spawn tsunamis, or even the water displaced from a vessels hull moving through the water.
While waves traveling within the depths of the ocean are longitudinal waves, the waves which travel along the surface of the oceans are referred to as surface waves. A surface wave is one in which particles of the medium undergo a circular motion. Surface waves are neither longitudinal nor transverse. In longitudinal and transverse waves, all the particles in the entire bulk of the medium move in a parallel and a perpendicular direction relative to the direction of energy transport. In a surface wave, it is only the particles at the surface breaking free which undergo the circular motion since the equilibrium of movement of the mass of the water seeks such, and a circular motion is always the best way to get there.
Waves seem to move through an ocean or lake, yet the water always returns to its rest position as the energy is transported through the medium, while the water molecules are not transported. Proof of this is the fact that there is still water in the middle of the ocean despite all the waves out there. The water has not moved from the middle of the ocean to the shore, just the energy which, when it finally hits the shore, is expended by actually moving the water up the beach or splashing and releasing the energy across a rocky shore.
The reason the energy in the wave changes its transport behavior close to shore is that as the wave approaches shallower water, the medium that is the water is altered —restricted actually — and it starts compressing and concentrating the energy, hindering it from making the full undulations and oscillations it enjoyed at greater depths and then channeling that concentrated energy, magnifying it’s power, often with quite erosive or even destructive force.
So stretch your arms, limber your knees, muster up a little enthusiasm then gather the crew to get out on the water and celebrate and trade some wave energy!