The Nature of Waves

Discussion

The basics

definition

A wave is a disturbance that propagates through a medium. There are three words in that definition that may need unpacking: disturbance, propagate, and medium.

• A disturbance, in the sense used in this definition, is a change from the current state of a measurable quantity at some location. For example…
  • a change in a kinematic variable like position, velocity, or acceleration;
  • a change in an intensive property like pressure, density, or temperature;
  • a change in field strength like electric field strength, magnetic field strength, or gravitational field strength.
• To propagate, in the sense used in this definition, is to transmit the influence of something in a particular direction. Synonyms for propagate include spread, transmit, communicate, and broadcast. The noun form of the word is propagation.
• A medium is the substance through which a wave can propagate. Water is the medium of ocean waves. Air is the medium through which we hear sound waves. The electric and magnetic fields are the medium of light. People are the medium of a stadium wave. The Earth is the medium of seismic waves (earthquake waves). Cell membranes are the medium of nerve impulses. Transmission lines are the medium of alternating current electric power. Medium is the singuar form of the noun. Media is the plural form (although mediums is prefered by some people).

Let's list a few key examples of wave phenomena and then connect them to this definition. The first example that comes to mind when most people hear the word wave are the kinds of waves that one sees on the surface of a body of water: deep water waves in the ocean or ripples in a puddle. The most important kinds of waves for humans are the waves we use to sense the world around us: sound and light.

Imagine a calm pool. The surface is flat and smooth. Drop a rock into it. Kerploop. The surface is now disturbed. It is higher than normal in some places and lower than normal in others. The disturbed water at the point of impact disturbs the water next to it, which in turn disturbs the water next to it, which disturbs the water next to it, and so on. The disturbance spreads outward, transmits, or propagates. The medium through which this disturbance propagates is the surface of the water.

Imagine a quiet room. The air inside is still. Drop a book onto a table in that room. Thwap. The air between the book and the table is squeezed out in a fraction of a second. The air pressure in that rapidly decreasing gap rises above normal and then rebounds. The rise and fall of pressure is like the rise and fall of the surface of the pool in the previous example. The air under the book bumps the air on the edges of the book, which bumps the air next to it, which bumps the air next to it, and so on. The medium through which this disturbance propagates is the air.

Those were the easy examples. Water waves and sound waves are examples of mechanical waves — waves that propagate through a material medium. Light is not so easy to understand as a wave, which is why there are multiple sections of this book devoted to it. Still, I am going to try to describe it briefly.

Imagine a dark cavern, deep within the Earth. The electric and magnetic fields inside are relatively static and unchanging. Strike a match. Skeerach. The atoms of carbon in the wood of the matchstick combine with the atoms of oxygen in the air releasing heat. The heat agitates the atoms of the combustion products resulting in the phenomenon known as fire. The electrons bound to the rapidly vibrating atoms disturb the electric and magnetic fields in the space surrounding them. These fields are "elastic" in a sense. A wiggle in the fields in one place causes a wiggle in the fields nearby, which causes a wiggle in the fields nearby, and so on. These wiggles eventually make it to your eye, which you perceive as light. The electric and magnetic fields that fill all of space are the medium.

essential property

Waves transfer energy, momentum, and information, but not mass.

A naive description of a wave is that it has something to do with motion. But the motion of a wave on the water is not the same as the motion of the water from a hose. When waves move over the surface of the ocean, where does the ocean go? Nowhere. When waves reach the shore, does the water accumulate into great heaps? No. The water moves in and out, and the ocean stays behind. Even when huge tsunamis strike, the wall of water deposited on the land eventually drains back into the sea. In this case, no net transfer of mass has occurred.

Compare this to the water from a hose. The water comes pouring out the open end and stays where it lands forming a puddle or drains away to some other location. It most certainly does not jump back into the hose. In this case, mass has been transferred from one location to another.

Any sensible person who owns waterfront property should be familiar with the word erosion. Ocean waves (or waves on the Great Lakes for that matter) break on the shore, beating the rock and soil into submission and pulling it away. This material will never return. (If there were no plate tectonic forces lifting the land up in some places or volcanoes creating new land in others, the Earth would be covered in a global sea of uniform depth.) A force (F) has been exerted and mass has been displaced (∆s). Work has been done (W = F∆s). The ability to do work is one definition of energy (W = ∆E). Thus waves transfer energy.

Sticking with the example of ocean waves, anyone who surfs knows that waves transfer momentum. I have less to say on this subject.

Sound and light are the two primary examples of the way we gather information around us as humans. We have specialized sensory organs called ears and eyes for doing just that.

examples

Here's a list of some phenomena or activities that satisfy the definition of a wave given above.

• Sound waves
  • including hock waves
• Light waves and all the other forms of electromagnetic radiation
  • Radio waves
  • Microwaves
  • Infrared
  • Visible light
  • Ultraviolet
  • X-rays
  • Gamma rays
• Water waves
  • Deep water waves (gravity waves, ocean waves, wind waves)
    • Tsunamis (tidal waves)
  • Ripples (capillary waves)
• Seismic waves, a.k.a. earthquake waves
  • P waves (primary waves, pressure waves)
  • S waves (secondary waves, shear waves)
  • R waves (Rayleigh waves, ground roll)
  • L waves (Love waves)
• Waves in cloth
  • A fluttering flag
  • Snapping a sheet when making a bed
• Physiological waves
  • Nerve impulses
  • Peristalsis
  • Heart contractions
• Animal locomotion
  • Snakes, eels, etc
  • Worms, slugs, etc.
  • Centipedes, millipedes, caterpillars, etc.
• Waves in linear media
  • Plucking, bowing, or striking a guitar, violin, or piano string
  • Casting loops when fly fishing
  • Cracking a whip
• Gravitational waves (as described in general relativity, not to be confused with gravity waves in water)
• Matter waves (quantum mechanical waves, de Broglie waves)
• Miscellaneous
  • Dominoes (as a show, not as a game)
  • Stadium wave (Mexican wave, the wave)
  • Newton's cradle (paid link)

not examples

Just because the word wave is used doesn't mean the thing being described is a wave in the sense used in this book.

• Waving as a signal to get someone's attention or to greet them or to say goodbye is not a wave. It does not propagate in a direction. Just because you wave at me does not mean that I have to start waving followed by a person behind me and the person behind them and the person behind them.
• A permanent wave set in a person's hair is not a wave. The term is almost an oxymoron. Nothing's moving if a thing is permanent. Also, you getting a wave set in your hair does not result in the people nearest you getting one followed by the people next to them getting one, and so on, until the whole globe is filled with wavy haired people.
• Wheat, or any other tall grass, is sometimes said to wave when gusts of wind pass over it. That bulk flow of matter is not a wave and neither is the response of the wheat.
• A heat wave is a meteorological term referring to a prolonged period of unusually high temperature. This definition has no connection to a phenomenon that propagates. Just because it's hot for a long period in one location does not imply that the heat wave will propagate to another location. It's actually sort of the opposite. A heat wave is often a region where the hot air is "stuck". The opposite of a heat wave could be called a cold wave, but it's usually described a cold snap (at least in the dialect of English I'm used to hearing). This should indicate that neither one of these phenomena is really a wave. Sometimes infrared radiation is described colloquially as heat waves, but that's not the right term. The same goes for rising thermals in the desert. That shimmering effect sometimes seen on the horizon is turbulent air of different density streaming upward and not a wave. Waves do not transfer mass.
• A crime wave is like a heat wave, but for crime. Since heat waves aren't waves, neither are crime waves.
• Making waves, meaning to stir up trouble, is not an example of a wave — and don't you disagree with me you trouble maker.

Classification of waves

by medium

Waves can be classified according to the medium through which they propagate.

mechanical waves

…require a material medium. Sound is the most important example of a mechanical wave. Sound waves cannot travel through a vacuum.

electromagnetic waves

…propagate through the electric and magnetic fields that are everywhere in space. Light is the most important example of an electromagnetic wave for humans. Electromagnetic waves can propagate through transparent materials and can also propagate through empty space. The basic electromagnetic spectrum in order of increasing frequency is radio waves, microwaves, infrared, visible light, ultraviolet, x-rays, and gamma rays.

gravitational waves

…propagate through the gravitational field that is everywhere in space. Similar to electromagnetic waves, gravitational waves can propagate through matter or empty space. Gravitational waves are a consequence of Einstein's theory of general relativity and should not be confused with gravity waves, which is a name for the kind of waves found in deep water (a.k.a. ocean waves). Gravitational waves were predicted to exist in 1916 (or 1918) but were not confirmed with direct observation until 2015.

matter waves

…refer to the quantum mechanical description of fundamental particles like electrons and quarks as a wave. Each particle type has a wave equation that describes a particle field which can then be used to generate wave functions for the individual particles of that type. Squaring a wave function results in a probability distribution that describes the likelihood of finding a particle at a specific location. In a sense, mater waves are probability waves.

by the type of disturbance

Waves can be classified according to the type of disturbance — meaning its relative direction or shap. There is a lot that can be said about this organizational scheme. I'm starting this part of this section with a quick summary in table form followed by a rather detailed follow up.

transverse waves

A transverse wave is one in which the direction of the disturbance is perpendicular to the direction of propagation. The word transverse describes something pointing in a sideways or lateral direction. As dynamic phenomena, waves are better represented with animations than with static images. Click on the static image below to see a transverse wave in action.

Animate

A cartoon representation of this kind of wave is your classic wiggly line. People with a bit of math knowledge will tell you they drew a sine curve. Those with a little bit more math knowledge will say they drew a sine or cosine curve.

Magnify

The high parts on a curve like this are called crests. The low parts are called troughs. Since directions like up and down don't always make sense for waves, what the parts really represent are the maximal changes. The points labeled crests are points corresponding to a maximal increase of the changing quantity in a whatever direction is decided to be positive. The points labeled troughs are the points corresponding to the maximal change in the opposite direction.

Pronouncing words ending in -ough in English is often a mystery. The word trough rhymes with off. A trough is what one uses to provide food and water to livestock and other domesticated animals — typically a long, narrow open container that an animal would dip its head down into. The word crest rhymes with best. A crest is something at the top of something. Many birds, usually male birds, have crests. Hills and mountains are are sometimes said to have crests. The crest on a men's sports jacket or a school uniform gets its name from the heraldic crests that were originally worn on knight's helmets above the visor. Crests are up high. Troughs are down low.

The most important example of a transverse wave for humans is light. Most of what I am about to say in the following bullets will really be discussed elsewhere in this book.

• Showing that light is a just wave was not easy before the 20th century. Now that we have easy access to lasers in the 21st century, it's less of a problem. Light can be made to interfere with itself and produce a pattern of what are called interference fringes. A laser and two or more closely spaced openings are all that is needed. The iridescence seen when gasoline is spilled on water, in some insects like scarab beetles and butterflies, and in pearls and the shells of mollusks is caused by thin film interference. Observing this wave behavior of light requires no special technology.
• Showing that light is a transverse wave was was also not easy before the 20th century. Now that we have easy access to polarized sunglasses and polarized electronic displays in the 21st century, it's less of a problem. Light can be shown to exhibit polarization. Try looking at certain electronic displays with polarized sunglasses. If the orientation of the sunglasses is perpendicular to the orientation of the display, the display will look dark (or really screwed up). If the orientation is parallel, the display will look normal (or closer to normal than when they were perpendicular).

Lots of musical instruments make use of transverse waves to generate their characteristic sound.

• The source of the sound that comes out of violins, guitars, pianos and other chordophones is the side to side motion of a nylon or metal string (or in the olden days, dried animal intestines). The parts of the string vibrate side to side, but the wave travels along the length of the string. These two directions are perpendicular, which makes the waves transverse. The vibrations of the string are also transmitted into the wooded bodies or sound boards of these instruments. These flat wooden parts are driven to flex in and out, but the energy propagates across the surface. The two directions here are also perpendicular.
• The source of the sound that comes out of drum heads, kazoos or other membranophones is the in and out vibration of some flat, membrane-like structure. The waves produced by striking, stroking, or humming into these devices generates waves that crisscross the object. Once again, the disturbance is perpendicular to the propagation.
• Most percussion instruments that are not drums are classified as idiophones. Cymbals, triangles, and xylophones produce sound by the vibration of the entire object or a piece that is not a string, membrane, or column of air. The waves set up in many of these instruments are transverse, but because the class is so large and varied there is probably an exception out there somewhere.

Some animals propel themselves by sending transverse waves down the length of their bodies.

• Fish use a variety of means for getting around but long, thin, tubular fish like eels, lamprey, and dogfish are what comes to mind when I think about locomotion by transverse waves. A wave of side to side motion starts at the head and propagates backward along the spine. This propels the fish forward.
• Snakes also have several mechanisms for propelling themselves. The one that is considered most "normal" is called lateral undulation and has the classic look of a transverse wave in a one-dimensional medium. Much like the fish described above, a wave starts at the head as side to side motion and propagates backward down the length of the snake. A fancier kind of locomotion that relies on transverse waves is called sidewinding and the snakes that use it live in sandy deserts or slippery mud flats — anywhere getting a good grip on the ground is difficult. It's still an example of a transverse wave, but it propels the snake diagonally instead of forward relative to its body axis.

longitudinal waves

• longitudinal
  • synonyms
    • pressure, compression, density
  • examples
    • sound
    • musical instruments
      • aerophones - vibrating columns of air (horns, whistles, organ pipes )
    • seismic
      • primary (P) pressure
    • animal locomotion
      • invertebrates (worms)
    • traffic jams
    • density waves in spiral galaxies generate the arms
  • parts
    • compressions (a.k.a. condensation): the pressed part, the greatest positive pressure change, a region where the medium is under compression
    • rarefactions (a.k.a. dilations): the stretched part, the lowest negative pressure change, a region where the medium is under tension

Animate

Magnify

complex waves

classed by orientation of change

• surface, interface, complex
  • ocean
  • seismic waves come in two basic families
    • body waves, which have already been discussed
      • Primary (surface, compression, Pressure)
      • Secondary (transverse, Shear), can't propagate through liquids
    • surface waves, which are what this discussion is a part of
      • Love, (Lateral shear)
      • Rayleigh, (elliptical, plate waves, ground Roll), something like ocean waves, but elliptical instead of circular

Animate

torsional waves

Animate

by duration

classified by duration

• episodic
• periodic

Magnify

by appearance

Waves can be classified according to what they appear to be doing.

traveling waves

…are waves that appear to be propagating. This might seem like a distinction made by the Department of Redundancy Department, since propagation is a key part of the definition of a wave, but there actually are waves that do not appear to be going anywhere.

standing waves

…are waves that do not appear to be propagating. (They are also called stationary waves.) This appearance is actually an illusion that arises when a wave meets its own reflection under the right circumstances. A more complete discussion of this phenomenon appears in a in a separate section of this book.

Now look at these pretty, moving pictures.