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# INTERFERENCE

## CONCEPT

When two or more waves interact and combine, they interfere with one another. But interference is not necessarily bad: waves may interfere constructively, resulting in a wave larger than the original waves. Or, they may interfere destructively, combining in such a way that they form a wave smaller than the original ones. Even so, destructive interference may have positive effects: without the application of destructive interference to the muffler on an automobile exhaust system, for instance, noise pollution from cars would be far worse than it is. Other examples of interference, both constructive and destructive, can be found wherever there are waves: in water, in sound, in light.

## HOW IT WORKS

### Waves

Whenever energy ripples through space, there is a wave. In fact, wave motion can be defined as a type of harmonic motion (repeated movement of a particle about a position of equilibrium, or balance) that carries energy from one place to another without actually moving any matter. A wave on the ocean is an example of a mechanical wave, or one that involves matter; but though the matter moves in place, it is only the energy in the wave that experiences net movement.

Wave motion is related to oscillation, a type of harmonic motion in one or more dimensions. There is one critical difference, however: oscillation involves no net movement, only movement in place, whereas the harmonic motion of waves carries energy from one place to another. Yet, individual waves themselves are oscillating even as the overall wave pattern moves.

A transverse wave forms a regular up-and-down pattern in which the oscillation is perpendicular to the direction the wave is moving. Ocean waves are transverse, though they also have properties of longitudinal waves. In a longitudinal wave, of which a sound wave is the best example, oscillation occurs in the same direction as the wave itself.

#### PARAMETERS OF WAVE MOTION.

Some waves, composed of pulses, do not follow regular patterns. However, the waves of principal concern in the present context are periodic waves, ones in which a uniform series of crests and troughs follow each other in regular succession. Periodic motion is movement repeated at regular intervals called periods. In the case of wave motion, a period (represented by the symbol T ) is the amount of time required to complete one full cycle of the wave, from trough to crest and back to trough.

Period can be mathematically related to several other aspects of wave motion, including wave speed, frequency, and wavelength. Frequency (abbreviated f ) is the number of waves passing through a given point during the interval of one second. It is measured in Hertz (Hz), named after nineteenth-century German physicist Heinrich Rudolf Hertz (1857-1894), and a Hertz is equal to one cycle of oscillation per second. Higher frequencies are expressed in terms of kilohertz (kHz; 103 or 1,000 cycles per second) or megahertz (MHz; 106 or 1 million cycles per second.) Wavelength (represented by the symbol abbreviated λ, the Greek letter lambda) is the distance between a crest and the adjacent crest, or a trough and an adjacent trough, of a wave. The higher the frequency, the shorter the wavelength.

Another parameter for describing wave motionone that is mathematically independent from the quantities so far describedis amplitude, or the maximum displacement of particles from a position of stable equilibrium. For an ocean wave, amplitude is the distance from either the crest or the trough to the level that the ocean would maintain if it were perfectly still.

### Superposition and Interference

#### SUPERPOSITION.

The principle of superposition holds that when several individual but similar physical events occur in close proximity, the resulting effect is the sum of the magnitude of the separate events. This is akin to the popular expression, "The whole is greater than the sum of the parts," and it has numerous applications in physics.

Where the strength of a gravitational field is being measured, for instance, superposition dictates that the strength of that field at any given point is the sum of the mass of the individual particles in that field. In the realm of electromagnetic force, the same statement applies, though the units being added are electrical charges or magnetic poles, rather than quantities of mass. Likewise, in an electrical circuit, the total current or voltage is the sum of the individual currents and voltages in that circuit.

Superposition applies only in equations for linear eventsthat is, phenomena that involve movement along a straight line. Waves are linear phenomena, and, thus, the principle describes the behavior of all waves when they come into contact with one another. If two or more waves enter the same region of space at the same time, then, at any instant, the total disturbance produced by the waves at any point is equal to the sum of the disturbances produced by the individual waves.

#### INTERFERENCE.

The principle of superposition does not require that waves actually combine; rather, the net effect is as though they were combined. The actual combination or joining of two or more waves at a given point in space is called interference, and, as a result, the waves produce a single wave whose properties are determined by the properties of the individual waves.

If two waves of the same wavelength occupy the same space in such a way that their crests and troughs align, the wave they produce will have an amplitude greater than that possessed by either wave initially. This is known as constructive interference. The more closely the waves are in phasethat is, perfectly alignedthe more constructive the interference.

It is also possible that two or more waves can come together such that the trough of one meets the crest of the other, or vice versa. In this case, what happens is destructive interference, and the resulting amplitude is the difference between the values for the individual waves. If the waves are perfectly unalignedin other words, if the trough of one exactly meets the crest of the othertheir amplitudes cancel out, and the result is no wave at all.

### Resonance

It is easy to confuse interference with resonance, and, therefore, a word should be said about the latter phenomenon. The term resonance describes a situation in which force is applied to an oscillator at the point of maximum amplitude. In this way, the motion of the outside force is perfectly matched to that of the oscillator, making possible a transfer of energy. As with interference, resonance implies alignment between two physical entities; however, there are several important differences.

Resonance can involve waves, as, for instance, when sound waves resonate with the vibrations of an oscillator, causing a transfer of energy that sometimes produces dramatic results. (See essay on Resonance.) But in these cases, a wave is interacting with an oscillator, not a wave with a wave, as in situations of interference. Furthermore, whereas resonance entails a transfer of energy, interference involves a combination of energy.

#### TRANSFER VS. COMBINATION.

The importance of this distinction is easy to see if one substitutes money for energy, and people for objects. If one passes on a sum of money to another person, a business, or an institutionas a loan, repayment of a loan, a purchase, or a giftthis is an example of a transfer. On the other hand, when married spouses each earn paychecks, their cash is combined.

Transfer thus indicates that the original holder of the cash (or energy) no longer has it. Yet, if the holder of the cash combines funds with those of another, both share rights to an amount of money greater than the amount each originally owned. This is analogous to constructive interference.

On the other hand, a husband and wife (or any other group of people who pool their cash) also share liabilities, and, thus, a married person may be subject to debt incurred by his or her spouse. If one spouse creates debt so great that the other spouse cannot earn enough to maintain the payments, this painful situation is analogous to destructive interference.

## REAL-LIFE APPLICATIONS

### Mechanical Waves

One of the easiest ways to observe interference is by watching the behavior of mechanical waves. Drop a stone into a still pond, and watch how its waves ripple: this, as with most waveforms in water, is an example of a surface wave, or one that displays aspects of both transverse and longitudinal wave motion. Thus, as the concentric circles of a longitudinal wave ripple outward in one dimension, there are also transverse movements along a plane perpendicular to that of the longitudinal wave.

While the first wave is still rippling across the water, drop another stone close to the place where the first one was dropped. Now, there are two surface waves, crests and troughs colliding and interfering. In some places, they will interfere constructively, producing a waveor rather, a portion of a wavethat is greater in amplitude than either of the original waves. At other places, there will be destructive interference, with some waves so perfectly out of phase that at one instant in time, a given spot on the water may look as though it had not been disturbed at all.

One of the interesting aspects of this interaction is the lack of uniformity in the instances of interference. As suggested in the preceding paragraph, it is usually not entire waves, but merely portions of waves, that interfere constructively or destructively. The result is that a seemingly simple eventdropping two stones into a still pondproduces a dazzling array of colliding circles, broken by outwardly undisturbed areas of destructive interference.

A similar phenomenon, though manifested by the interaction of geometric lines rather than concentric circles, occurs when two power boats pass each other on a lake. The first boat chops up the water, creating a wake that widens behind it: when seen from the air, the boat appears to be at the apex of a triangle whose sides are formed by rippling eddies of water.

Now, another boat passes through the wake of the first, only it is going in the opposite direction and producing its own ever-widening wake as it goes. As the waves from the two boats meet, some are in phase, but, more often than not, they are only partly in phase, or they possess differing wavelengths. Therefore, the waves at least partially cancel out one another in places, and in other places, reinforce one another. The result is an interesting patchwork of patterns when seen from the air.

### Sound Waves

#### IN TUNE AND OUT OF TUNE.

The relationships between musical notes can be intriguing, and though tastes in music vary, most people know when music is harmonious and when it is discordant. As discussed in the essay on frequency, this harmony or discord can be equated to the mathematical relationships between the frequencies of specific notes: the lower the numbers involved in the ratio, the more pleasing the sound.

The ratio between the frequency of middle C and that of its first harmonicthat is, the C note exactly one octave above itis a nice, clean 1:2. If one were to play a song in the key of C-which, on a piano, involves only the "white notes" C-D-E-F-G-A-Beverything should be perfectly harmonious and (presumably) pleasant to the ear. But what if the piano itself is out of tune? Or what if one key is out of tune with the others?

The result, for anyone who is not tone-deaf, produces an overall impression of unpleasantness: it might be a bit hard to identify the source of this discomfort, but it is clear that something is amiss. At best, an out-of-tune piano might sound like something that belonged in a saloon from an old Western; at worst, the sound of notes that do not match their accustomed frequencies can be positively grating.

#### HOW A TUNING FORK WORKS.

To rectify the situation, a professional piano tuner uses a tuning fork, an instrument that produces a single frequencysay, 264 Hz, which is the frequency of middle C. The piano tuner strikes the tuning fork, and at the same time strikes the appropriate key on the piano. If their frequencies are perfectly aligned, so is the sound of both; but, more likely, there will be interference, both constructive and destructive.

As time passesmeasured in seconds or even fractions of secondsthe sounds of the tuning fork and that of the piano key will alternate between constructive and destructive interference. In the case of constructive interference, their combined sound will become louder than the individual sounds of either; and when the interference is destructive, the sound of both together will be softer than that produced by either the fork or the key.

The piano tuner listens for these fluctuations of loudness, which are called beats, and adjusts the tension in the appropriate piano string until the beats disappear completely. As long as there are beats, the piano string and the tuning fork will produce together a frequency that is the average of the two: if, for instance, the out-of-tune middle C string vibrates at 262 Hz, the resulting frequency will be 263 Hz.

#### DIFFERENCE TONES.

Another interesting aspect of the interaction between notes is the "difference tone," created by discord, which the human ear perceives as a third tone. Though E and F are both part of the C scale, when struck together, the sound is highly discordant. In light of what was said above about ratios between frequencies, this dissonance is fitting, as the ratio here involves relatively high numbers15:16.

When two notes are struck together, they produce a combination tone, perceived by the human ear as a third tone. If the two notes are harmonious, the "third tone" is known as a summation tone, and is equal to the combined frequencies of the two notes. But if the combination is dissonant, as in the case of E and F, the third tone is known as a difference tone, equal to the difference in frequencies. Since an E note vibrates at 330 Hz, and an F note at 352 Hz, the resulting difference tone is equal to 22 Hz.

#### DESTRUCTIVE INTERFERENCE IN SOUND WAVES.

When music is played in a concert hall, it reverberates off the walls of the auditorium. Assuming the place is well designed acoustically, these bouncing sound waves will interfere constructively, and the auditorium comes alive with the sound of the music. In other situations, however, the sound waves may interfere destructively, and the result is a certain muffled deadness to the sound.

Clearly, in a music hall, destructive interference is a problem; but there are cases in which it can be a benefitsituations, that is, in which the purpose, indeed, is to deaden the sound. One example is an automobile muffler. A car's exhaust system makes a great deal of noise, and, thus, if a car does not have a proper muffler, it creates a great deal of noise pollution. A muffler counteracts this by producing a sound wave out of phase with that of the exhaust system; hence, it cancels out most of the noise.

Destructive interference can also be used to reduce sound in a room. Once again, a machine is calibrated to generate sound waves that are perfectly out of phase with the offending noisesay, the hum of another machine. The resulting effect conveys the impression that there is no noise in the room, though, in fact, the sound waves are still there; they have merely canceled each other out.

### Electromagnetic Waves

In 1801, English physicist Thomas Young (1773-1829), known for Young's modulus of elasticity became the first scientist to identify interference in light waves. Challenging the corpuscular theory of light put forward by Sir Isaac Newton (1642-1727), Young set up an experiment in which a beam of light passed through two closely spaced pinholes onto a screen. If light was truly made of particles, he said, the beams would project two distinct points onto the screen. Instead, what he saw was a pattern of interference.

In fact, Newton was partly right, but Young's discovery helped advance the view of light as a wave, which is also partly right. (According to quantum theory, developed in the twentieth century, light behaves both as waves and as particles.) The interference in the visible spectrum that Young witnessed was manifested as bright and dark bands. These bands are known as fringesvariations in intensity not unlike the beats created in some instances of sound interference, described above.

#### OILY FILMS AND RAINBOWS.

Many people have noticed the strangely beautiful pattern of colors generated when light interacts with an oily substance, as when light reflected on a soap bubble produces an astonishing array of shades. Sometimes, this can happen in situations not otherwise aesthetically pleasing: an oily film in a parking lot, left there by a car's leaky crankcase, can produce a rainbow of colors if the sunlight hits it just right.

This happens because the thickness of the oil causes a delay in reflection of the light beam. Some colors pass through the film, becoming delayed and, thus, getting out of phase with the reflected light on the surface of the film. These shades destructively interfere to such an extent that the waves are cancelled, rendering them invisible. Other colors reflect off the surface so that they are perfectly in phase with the light traveling through the film, and appear as an attractive swirl of color on the surface of the oil.

The phenomenon of light-wave interference with oily or filmy surfaces has the effect of filtering light, and, thus, has a number of applications in areas relating to optics: sunglasses, lenses for binoculars or cameras, and even visors for astronauts. In each case, unfiltered light could be harmful or, at least, inconvenient for the user, and the destructive interference eliminates certain colors and unwanted reflections.

#### RADIO WAVES.

Visible light is only a small part of the electromagnetic spectrum, whose broad range of wave phenomena are, likewise, subject to constructive or destructive interference. After visible light, the area of the spectrum most people experience during an average day is the realm of relatively low-frequency, long-wave length radio waves and microwaves, the latter including television broadcast signals.

People who rely on an antenna for their TV reception are likely to experience interference at some point. However, an increasing number of Americans use either cable or satellite systems to pick up TV programs. These are much less susceptible to interference, due to the technology of coaxial cable, on the one hand, and digital compression, on the other. Thus, interference in television reception is a gradually diminishing problem.

Interference among radio signals continues to be a challenge, since most people still hear the radio via old-fashioned means rather than through new technology, such as the Internet. A number of interference problems are created by activity on the Sun, which has an enormously powerful electromagnetic field. Obviously, such interference is beyond the control of most radio listeners, but according to a Web page set up by WHKY Radio in Hickory, North Carolina, there are a number of things listeners can do to decrease interference in their own households.

Among the suggestions offered at the WHKY Web site is this: "Nine times out of ten, if your radio is near a computer, it will interfere with your radio. Computers send out all kinds of signals that your radio 'thinks' is a real radio signal. Try to locate your radio away from computers especially the monitor." The Web site listed a number of other household appliances, as well as outside phenomena such as power lines or thunderstorms, that can contribute to radio interference.

### WHERE TO LEARN MORE

Beiser, Arthur. Physics, 5th ed. Reading, MA: Addison-Wesley, 1991.

Bloomfield, Louis A. "How Things Work: Radio." How Things Work (Web site). <http://rabi.phys.virginia.edu/HTW//radio.html> (April 27, 2001).

Harrison, David. "Sound" (Web site). <http://www.newi.ac.uk/buckley/sound.html> (April 27, 2001).

Interference Handbook/Federal Communications Commission (Web site). <http://www.fcc.gov/cib/Publications/tvibook.html> (April 27, 2001).

Internet Resources for Sound and Light (Web site). <http://electro.sau.edu/SLResources.html> (April 25, 2001).

"LightA-to-Z Science." DiscoverySchool.com (Web site). <http://school.discovery.com/homeworkhelp/worldbook/atozscience/l/323260.html> (April 27, 2001).

Oxlade, Chris. Light and Sound. Des Plaines, IL: Heinemann Library, 2000.

"Sound WaveConstructive and Destructive Interference" (Web site). <http://csgrad.cs.vt.edu/~chin/interference.html> (April 27, 2001).

Topp, Patricia. This Strange Quantum World and You. Nevada City, CA: Blue Dolphin, 1999.

WHKY Radio and TV (Web site). <http://www.whky.com/antenna.html> (April 27, 2001).

## KEY TERMS

### AMPLITUDE:

The maximum displacement of particles in oscillation from a position of stable equilibrium. For an ocean wave, amplitude is the distance from either the crest or the trough to the level that the ocean would maintain if it were perfectly still.

### CONSTRUCTIVE INTERFERENCE:

A type of interference that occurs when two or more waves combine in such a way that they produce a wave whose amplitude is greater than that of the original waves. If waves are perfectly in phasein other words, if the crest and trough of one exactly meets the crest and trough of the otherthen the resulting amplitude is the sum of the individual amplitudes of the separate waves.

### CYCLE:

In oscillation, a cycle occurs when the oscillating particle moves from a certain point in a certain direction, then switches direction and moves back to the original point. Typically, this is from the position of stable equilibrium to maximum displacement and back again to the stable equilibrium position. In a wave, a cycle is equivalent to the movement from trough to crest and back to trough.

### DESTRUCTIVE INTERFERENCE:

A type of interference that occurs when two or more waves combine to produce a wavewhose amplitude is less than that of the original waves. If waves are perfectly out of phasein other words, if the trough of one exactly meets the crest of the other, and vice versatheir amplitudes cancelout, and the result is no wave at all.

### FREQUENCY:

In wave motion, frequency is the number of waves passing through a given point during the interval of one second. The higher the frequency, the shorter the wavelength. Frequency is mathematically related to wave speed and period.

### HARMONIC MOTION:

The repeated movement of a particle about a position of equilibrium, or balance.

### HERTZ:

A unit for measuring frequency, named after nineteenth-century German physicist Heinrich Rudolf Hertz (1857-1894). High frequencies are expressed in terms of kilohertz (kHz; 103 or 1,000 cycles per second) or megahertz (MHz; 106 or 1 million cycles per second.)

### INTERFERENCE:

The combination of two or more waves at a given point in space to produce a wave whose properties are determined by the properties of the individual waves. This accords with the principle of superposition.

### LONGITUDINAL WAVE:

A wave in which the movement of vibration is in the same direction as the wave itself. This is contrasted to a transverse wave.

### MAXIMUM DISPLACEMENT:

For an object in oscillation, maximum displacement is the furthest point from stable equilibrium.

### MECHANICAL WAVE:

A type of wavefor example, a wave on the oceanthat involves matter. The matter itself may move in place, but as with all types of wave motion, there is no net movement of matteronly of energy.

### OSCILLATION:

A type of harmonic motion, typically periodic, in one or more dimensions.

### PERIOD:

For wave motion, a period is the amount of time required to complete one full cycle. Period is mathematically related to frequency, wavelength, and wave speed.

### PERIODIC MOTION:

Motion that is repeated at regular intervals. These intervals are known as periods.

### PERIODIC WAVE:

A wave in which a uniform series of crests and troughs follow one after the other in regular succession.

### PHASE:

When two waves of the same frequency and amplitude are perfectly aligned, they are said to be in phase.

### PRINCIPLE OF SUPERPOSITION:

A physical principle stating that when several individual, but similar, physical events occur in close proximity to one another, the resulting effect is the sum of the magnitude of the separate events. Interference is an example of superposition.

### PULSE:

An isolated, non-periodic disturbance that takes place in wave motion of a type other than that of a periodic wave.

### RESONANCE:

The condition in which force is applied to an object in oscillation at the point of maximum amplitude.

### STABLE EQUILIBRIUM:

A position in which, if an object were disturbed, it would tend to return to its original position. For an object in oscillation, stable equilibrium is in the middle of a cycle, between two points of maximum displacement.

### SURFACE WAVE:

A wave that exhibits the behavior of both a transverse wave and a longitudinal wave.

### TRANSVERSE WAVE:

A wave in which the vibration or motion is perpendicular to the direction in which the wave is moving. This is contrasted to a longitudinal wave.

### WAVELENGTH:

The distance between a crest and the adjacent crest, or the trough and an adjacent trough, of a wave. Wavelength, abbreviated λ (the Greek letter lambda) is mathematically related to wave speed, period, and frequency.

### WAVE MOTION:

A type of harmonic motion that carries energy from one place to another, without actually moving anymatter.

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## interference

interference, in physics, the effect produced by the combination or superposition of two systems of waves, in which these waves reinforce, neutralize, or in other ways interfere with each other. Interference is observed in both sound waves and electromagnetic waves, especially those of visible light and radio.

Interference in Sound Waves

When two sound waves occur at the same time and are in the same phase, i.e., when the condensations of the two coincide and hence their rarefactions also, the waves reinforce each other and the sound becomes louder. This is known as constructive interference. On the other hand, two sound waves occurring simultaneously and having the same intensity neutralize each other if the rarefactions of the one coincide with the condensations of the other, i.e., if they are of opposite phase. This canceling is known as destructive interference. In this case, the result is silence.

Alternate reinforcement and neutralization (or weakening) take place when two sound waves differing slightly in frequency are superimposed. The audible result is a series of pulsations or, as these pulsations are called commonly, beats, caused by the alternate coincidence of first a condensation of the one wave with a condensation of the other and then a condensation with a rarefaction. The beat frequency is equal to the difference between the frequencies of the interfering sound waves.

Interference in Light Waves

Light waves reinforce or neutralize each other in very much the same way as sound waves. If, for example, two light waves each of one color (monochromatic waves), of the same amplitude, and of the same frequency are combined, the interference they exhibit is characterized by so-called fringes—a series of light bands (resulting from reinforcement) alternating with dark bands (caused by neutralization). Such a pattern is formed either by light passing through two narrow slits and being diffracted (see diffraction), or by light passing through a single slit. In the case of two slits, each slit acts as a light source, producing two sets of waves that may combine or cancel depending upon their phase relationship. In the case of a single slit, each point within the slit acts as a light source. In all cases, for light waves to demonstrate such behavior, they must emanate from the same source; light from distinct sources has too many random differences to permit interference patterns.

The relative positions of light and dark lines depend upon the wavelength of the light, among other factors. Thus, if white light, which is made up of all colors, is used instead of monochromatic light, bands of color are formed because each color, or wavelength, is reinforced at a different position. This fact is utilized in the diffraction grating, which forms a spectrum by diffraction and interference of a beam of light incident on it. Newton's rings also are the result of the interference of light. They are formed concentrically around the point of contact between a glass plate and a slightly convex lens set upon it or between two lenses pressed together; they consist of bright rings separated by dark ones when monochromatic light is used, or of alternate spectrum-colored and black rings when white light is used. Various natural phenomena are the result of interference, e.g., the colors appearing in soap bubbles and the iridescence of mother-of-pearl and other substances.

Interference as a Scientific Tool

The experiments of Thomas Young first illustrated interference and definitely pointed the way to a wave theory of light. A. J. Fresnel's experiments clearly demonstrated that the interference phenomena could be explained adequately only upon the basis of a wave theory. The thickness of a very thin film such as the soap-bubble wall can be measured by an instrument called the interferometer. When the wavelength of the light is known, the interferometer indicates the thickness of the film by the interference patterns it forms. The reverse process, i.e., the measurement of the length of an unknown light wave, can also be carried out by the interferometer.

The Michelson interferometer used in the Michelson-Morley experiment of 1887 to determine the velocity of light had a half-silvered mirror to split an incident beam of light into two parts at right angles to one another. The two halves of the beam were then reflected off mirrors and rejoined. Any difference in the speed of light along the paths could be detected by the interference pattern. The failure of the experiment to detect any such difference threw doubt on the existence of the ether and thus paved the way for the special theory of relativity.

Another type of interferometer devised by Michelson has been applied in measuring the diameters of certain stars. The radio interferometer consists of two or more radio telescopes separated by fairly large distances (necessary because radio waves are much longer than light waves) and is used to pinpoint and study various celestial sources of radiation in the radio range. Astronomical interferometers consisting of two or more optical telescopes are used to enhance visible images of distant celestial objects. See radio astronomy; virtual telescope.

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# Interference

Interference is the interaction of two or more waves. Wave motion is a common phenomenon in everyday life. Light and sound, for example, are transmitted by waves. In addition, waves can often be seen on lakes, ponds, and other bodies of water.

All waves have high points, called crests, and low points, called troughs (pronounced trawfs). Suppose that two or more waves are generated at the same time, as shown in the accompanying photograph. Here, water waves are spreading out from the point where pebbles have been dropped into a pond. You can see how the waves overlap each other at various points on the surface of the water. This overlapping effect is interference.

## Constructive and destructive interference

In general, waves can interfere with each other in one of two ways: constructively or destructively. When the crests of two waves and the troughs of two waves arrive at a given spot at the same time, their effects are added to each other. The result is constructive interference. When the crest of one wave and the trough of a second wave arrive at the same time, their effects cancel each other out. The result is destructive interference.

Interference of sound waves. Constructive and destructive interference can be detected by the intensity of the result. For example, suppose that two sound waves interfere with each other constructively. In that case, the sound is louder than is the case for either wave individually. If the two sound waves interfere destructively, the sound is more quiet than with either wave individually.

Interference of light waves. Interference of light waves has been studied for many years. It was first described in 1801 by English physician and physicist (one who studies the science of matter and energy) Thomas Young (17731829). Young found that light waves can be made to interfere in such a way as to produce bright and dark bands called fringes.

Interference also accounts for the range of colors (called a rainbow or spectrum) sometimes produced by reflected light. When white light from the Sun reflects off a thin film of oil, interference may occur. Light of some colors is reflected off the top of the film. Light of other colors is reflected off the bottom of the film. The two sets of reflected light interfere with each other either constructively or destructively. Constructive interference results in the production of bright colors of different shades. Destructive interference produces dark bands with no color.

## Applications

Modern technology makes use of interference in many ways. Some experimental automobile mufflers listen for the sound wave produced in

the exhaust system. The muffler then produces another sound wave that is out of phase with the exhaust sound. The two waves interfere destructively, canceling the noise that would otherwise be produced by the exhaust system.

The oil film phenomenon described earlier is used for filtering light. Precise coatings on optical lenses in binoculars or cameras, astronaut's visors, or even sun glasses cause destructive interference that eliminates certain unwanted colors or stray reflections.

[See also Diffraction; Interferometry; Wave motion ]

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## interference

in·ter·fer·ence / ˌintərˈfi(ə)rəns/ • n. 1. the action of interfering or the process of being interfered with: he denied that there had been any interference in the country's internal affairs | an unwarranted interference with personal liberty. ∎ Football the action of illegally interfering with an opponent's ability to catch a passed or kicked ball. ∎  Football the legal blocking of an opponent or opponents to clear a way for the ballcarrier. ∎  Baseball any of various forms of hindering a player's ability to make a play, run, hit, etc. ∎  (in ice hockey and other sports) the illegal hindering of an opponent not in possession of the puck or ball. 2. Physics the combination of two or more electromagnetic waveforms to form a resultant wave in which the displacement is either reinforced or canceled. ∎  the fading or disturbance of received radio signals caused by unwanted signals from other sources, such as unshielded electrical equipment, or broadcasts from other stations or channels. PHRASES: run interference Football move in such a way as to provide legal interference (see sense 1 above). ∎ inf. intervene on someone's behalf, typically so as to protect them from distraction or annoyance: Elizabeth was quick to run interference and said that the professor would be very busy. DERIVATIVES: in·ter·fe·ren·tial / -fəˈrenchəl/ adj.

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"interference." The Oxford Pocket Dictionary of Current English. . Encyclopedia.com. 22 May. 2017 <http://www.encyclopedia.com>.

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"interference." The Oxford Pocket Dictionary of Current English. . Retrieved May 22, 2017 from Encyclopedia.com: http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/interference-0

## INTERFERENCE

INTERFERENCE. In LINGUISTICS and LANGUAGE TEACHING, the effect of one language on another, producing ‘instances of deviation from the norms of either language’ ( Uriel Weinreich, Languages in Contact, 1953). Interference occurs naturally in the speech of bi- and multilingual people and the efforts of learners of foreign languages. It affects all levels of language: accent, pronunciation, syntax, morphology, vocabulary, and idiom. Although it is natural, especially when speakers are tired, tense, excited, or distracted, it is often disapproved of in unilingual societies as a display of inadequate skill on the part of a foreign learner. See INTERLANGUAGE, MISTAKE.

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"INTERFERENCE." Concise Oxford Companion to the English Language. . Encyclopedia.com. 22 May. 2017 <http://www.encyclopedia.com>.

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"INTERFERENCE." Concise Oxford Companion to the English Language. . Retrieved May 22, 2017 from Encyclopedia.com: http://www.encyclopedia.com/humanities/encyclopedias-almanacs-transcripts-and-maps/interference

# INTERFERENCE

In the law ofpatents, the presence of two pending applications, or an existing patent and a pending application that encompass an identical invention or discovery.

When interference exists, the patent and trademark office conducts an investigation to ascertain the priority of invention between the conflicting applications, or the application and the patent. A patent is customarily granted to the earlier invention.

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"Interference." West's Encyclopedia of American Law. . Encyclopedia.com. 22 May. 2017 <http://www.encyclopedia.com>.

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"Interference." West's Encyclopedia of American Law. . Retrieved May 22, 2017 from Encyclopedia.com: http://www.encyclopedia.com/law/encyclopedias-almanacs-transcripts-and-maps/interference

## interference

interference In optics, the interaction of two or more wave motions, such as those of light and sound, creating a disturbance pattern. Constructive interference is the reinforcement of the wave motion because the component motions are in phase. Interference is used in the creation of 3-D holography. Destructive interference occurs when two waves are out of phase and cancel each other.

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"interference." World Encyclopedia. . Encyclopedia.com. 22 May. 2017 <http://www.encyclopedia.com>.

"interference." World Encyclopedia. . Encyclopedia.com. (May 22, 2017). http://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/interference

"interference." World Encyclopedia. . Retrieved May 22, 2017 from Encyclopedia.com: http://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/interference

## interference

interference The combination of waves. Constructive interference occurs when peaks add to peaks; destructive interference occurs when a peak coincides with and cancels out a trough.

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"interference." A Dictionary of Earth Sciences. . Encyclopedia.com. 22 May. 2017 <http://www.encyclopedia.com>.

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"interference." A Dictionary of Earth Sciences. . Retrieved May 22, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/interference

## interference

interference See mutual interference.

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"interference." A Dictionary of Ecology. . Encyclopedia.com. 22 May. 2017 <http://www.encyclopedia.com>.

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## interference

interferenceabeyance, conveyance, purveyance •creance • ambience •irradiance, radiance •expedience, obedience •audience •dalliance, mésalliance •salience •consilience, resilience •emollience • ebullience •convenience, lenience, provenience •impercipience, incipience, percipience •variance • experience •luxuriance, prurience •nescience • omniscience •insouciance • deviance •subservience • transience •alliance, appliance, compliance, defiance, misalliance, neuroscience, reliance, science •allowance •annoyance, clairvoyance, flamboyance •fluence, pursuance •perpetuance • affluence • effluence •mellifluence • confluence •congruence • issuance • continuance •disturbance •attendance, dependence, interdependence, resplendence, superintendence, tendance, transcendence •cadence •antecedence, credence, impedance •riddance • diffidence • confidence •accidence • precedence • dissidence •coincidence, incidence •evidence •improvidence, providence •residence •abidance, guidance, misguidance, subsidence •correspondence, despondence •accordance, concordance, discordance •avoidance, voidance •imprudence, jurisprudence, prudence •impudence • abundance • elegance •arrogance • extravagance •allegiance • indigence •counter-intelligence, intelligence •negligence • diligence • intransigence •exigence •divulgence, effulgence, indulgence, refulgence •convergence, divergence, emergence, insurgence, resurgence, submergence •significance •balance, counterbalance, imbalance, outbalance, valance •parlance • repellence • semblance •bivalence, covalence, surveillance, valence •sibilance • jubilance • vigilance •pestilence • silence • condolence •virulence • ambulance • crapulence •flatulence • feculence • petulance •opulence • fraudulence • corpulence •succulence, truculence •turbulence • violence • redolence •indolence • somnolence • excellence •insolence • nonchalance •benevolence, malevolence •ambivalence, equivalence •Clemence • vehemence •conformance, outperformance, performance •adamance • penance • ordinance •eminence • imminence •dominance, prominence •abstinence • maintenance •continence • countenance •sustenance •appurtenance, impertinence, pertinence •provenance • ordnance • repugnance •ordonnance • immanence •impermanence, permanence •assonance • dissonance • consonance •governance • resonance • threepence •halfpence • sixpence •comeuppance, tuppence, twopence •clarence, transparence •aberrance, deterrence, inherence, Terence •remembrance • entrance •Behrens, forbearance •fragrance • hindrance • recalcitrance •abhorrence, Florence, Lawrence, Lorentz •monstrance •concurrence, co-occurrence, occurrence, recurrence •encumbrance •adherence, appearance, clearance, coherence, interference, perseverance •assurance, durance, endurance, insurance •exuberance, protuberance •preponderance • transference •deference, preference, reference •difference • inference • conference •sufferance • circumference •belligerence • tolerance • ignorance •temperance • utterance • furtherance •irreverence, reverence, severance •deliverance • renascence • absence •acquiescence, adolescence, arborescence, coalescence, convalescence, deliquescence, effervescence, essence, evanescence, excrescence, florescence, fluorescence, incandescence, iridescence, juvenescence, luminescence, obsolescence, opalescence, phosphorescence, pubescence, putrescence, quiescence, quintessence, tumescence •obeisance, Renaissance •puissance •impuissance, reminiscence •beneficence, maleficence •magnificence, munificence •reconnaissance • concupiscence •reticence •licence, license •nonsense •nuisance, translucence •innocence • conversance • sentience •impatience, patience •conscience •repentance, sentence •acceptance • acquaintance •acquittance, admittance, intermittence, pittance, quittance, remittance •assistance, coexistence, consistence, distance, existence, insistence, outdistance, persistence, resistance, subsistence •instance • exorbitance •concomitance •impenitence, penitence •appetence •competence, omnicompetence •inheritance • capacitance • hesitance •Constance • importance • potence •conductance, inductance, reluctance •substance • circumstance •omnipotence • impotence •inadvertence • grievance •irrelevance, relevance •connivance, contrivance •observance • sequence • consequence •subsequence • eloquence •grandiloquence, magniloquence •brilliance • poignance •omnipresence, pleasance, presence •complaisance • malfeasance •incognizance, recognizance •usance • recusance

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