23 nde-ed.org/Physics/
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`Physics of Nondestructive Evaluation
`Atomic Elements
`
`Physics of Nondestructive Evaluation > Waves
`
`Waves
`
`Introduction to Waves
`
`Mechanical Wave Characteristics
`
`Characteristics of EM Radiation
`
`Wave Number and Impedance
`
`Attenuation of Waves
`
`WaveInteraction or Interference
`
`There are three primary categories of waves: mechanical, electromagnetic, and matter.
`® Introduction to Waves
`e Types of Waves
`.
`;
`;
`,
`Wavesand their propagation can be described by certain parameters.
`* tWechanicsl Wave Charactanicties
`
`® Characteristics of EM Radiation
`e Properties of Plane Waves
`¢ Wave Number and Impedance
`S Attar ae ies
`
`;
`;
`;
`Waves can interact with mediums and boundaries between mediums.
`* Wewe Intecschon er laterterence
`
`¢ Refraction and Snell's Law
`Refraction and Snell's Law
`Reflection and Transmission Coefficients
`Be
`ae
`Reflection and Transmission CoefficientslaaAaaaaa
`* Mode Conversion
`
`Mode Conversion
`
`Wavelength and Defect Detection
`Signal-to-Noise Ratio
`
`Different types of waves with different properties are useful for different types of NDT,
`Page 1 of 68
`e Wavelength and Defect Detection
`© Signal-to-Noise Ratio
`
`HAPTIC EX2004
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`
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`Atomic Elements
`
`Physics of Nondestructive Evaluation > Waves > Introduction to Waves
`
`Introduction to Waves
`
`Mechanical Wave Characteristics
`Characteristics of EM Radiation
`
`Wave Number and Impedance
`
`Attenuation of Waves
`
`WaveInteraction or Interference
`
`Refraction and Snell's Law
`Reflection and Transmission Coefficients
`
`Mode Conversion
`
`Wavelength and Defect Detection
`Signal-to-Noise Ratio
`
`Introduction to Waves
`
`After reading this section you will be able to do the following:
`e Name the three categories of waves.
`;
`;
`® Provide examples of mechanical and electromagnetic waves.
`
`There are three primary categories of waves:
`
`1. Mechanical
`2. Electromagnetic
`3. Matter
`
`In this module we'll discuss
`Each of these categories of waves breaks down into additional types of waves.
`mechanical waves and electromagnetic (EM) waves. Matter waves are a part of quantum theory and are
`therefore beyond the scope of this material. Some examples of mechanical and electromagnetic waves are
`shown below.
`In the mechanical waves column, we see ripples in a pond and sound. Other examples of
`mechanical waves are waves on the ocean and seismic waves that occur from earthquakes.
`In general,
`:
`mechanical waves are waves that require a medium to travel. In the electromagnetic waves column below we see
`the sun which produces ultraviolet radiation, a type of EM wave. We also see an antenna which produces EM
`waves at microwave frequencies. EM waves do not require a medium to travel. In the following pages we'll learn
`more about the different types of EM radiation, as well as other wave characteristics.
`
`¢
`
`Page 2 of 68
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`About
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`Physics
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`Glossary
`
`
`Atomic Elements
`Mechanical Waves
`Electromagnetic Waves
`
`
`
`Introduction to Waves
`
`Mechanical Wave Characteristics
`
`Characteristics of EM Radiation
`
`o
`
`oe
`
` -
`
`
`
`SP J
`
`
`
`Plane Waves
`
`Review:
`1. There are 3 primary categories of waves: mechanical, electramagnetic (EM), and matter.
`2. Mechanical waves require a medium to travel while EM waves do not.
`
`Wave Number and Impedance
`Attenuation of Waves
`WaveInteraction or Interference
`
`Refraction and Snell's Law
`
`Reflection and Transmission Coefficients
`
`Mode Conversion
`
`Wavelength and Defect Detection
`
`Signal-to-Noise Ratio
`
`.
`
`Physics of Nondestructive Evaluation > Waves > Types of Waves
`
`Page 3 of 68
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`IOWA STATE UNIVERSITY
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`About
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`Physics
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`NDEEngineering
`
`Glossary
`
`
`
`Atomic Elements
`
`Types of Waves
`
`Introduction to Waves
`
`Mechanical Wave Characteristics
`
`Characteristics of EM Radiation
`
`Wave Number and Impedance
`
`Attenuation of Waves
`
`Wave Interaction or Interference
`
`Refraction and Snell's Law
`
`Reflection and Transmission Coefficients
`
`After reading this section you will be able to do the following:
`e Describe the different types of mechanical waves.
`® Explain what electromagnetic wavesare.
`
`As previously discussed there are three primary categories of waves: mechanical, electromagnetic, and matter.
`Some more details on mechanical and electromagnetic waves will be discussed on this page. As a reminder, the
`big difference between mechanical waves and electromagnetic waves is that mechanical waves are disturbances
`that travel in a medium while electromagnetic waves do not require medium (they can travel in a vacuum!).
`
`Mechanical Waves
`
`While mechanical waves cause a disturbance in a medium, they do not transport matter as they travel. They only
`transport energy. Within the category of mechanical wavesthere are 3 primary types:transverse,longitudinal,
`and surface.
`
`Mode Conversion
`
`Transverse Waves
`
`Wavelength and Defect Detection
`
`Signal-ta-Noise Ratio
`
`Imagine you have a long string. You tie one end to a tree and hold the other end so the string is under tension.
`You then move the end of the string you're holding up and down and you see this movementtravel down the
`string towards the tree.
`In doing so, you're causing a displacement of the medium (the string)
`that
`is
`perpendicular/transverse to the direction of travel. In other words, you're causing a transverse wave!
`
`Page 4 of 68
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`Atomic Elements
`
`SS—————E—E—E—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—___
`Longitudinal Waves
`Longitudinalwavesalso called compressionwaves and pressure waves. In longitudinal wavesthe displacement
`of the medium is in the same direction as the wave travels. To visualize this, imagine liquid in a tube with a
`plunger or piston at one end of the tube and a rigid wall at the other end. When you push on the piston you're
`causing displacement and pressure fluctuations in the liquid (the medium) that travel down the tube. Another
`wayto visualize this is
`
`Introduction to Waves
`
`Surface Waves
`
` es travel along the ir
` ce between two different mediums. Some examples are ocean waves or
`eismic waves that often occur as a result of an earthquake.
`In surfacewaves the molecules of the medium
`undergo circular motion and are therefore neither transverse nor longitudinal. However, they can be described as
`having transverseand longitudinal components. Ground wavesare an electromagnetic version of surface waves,
`but are beyond the scopeofthis material.
`
`5 s
`
`Electromagnetic Waves
`
`
`In the electricity and magnetism modules we discussed electric fields and magnetic fields, respectively. We also
`briefly discussed how a changing magneticfield can produce an electric field and vice versa. It is these linked
`changing electric and magnetic fields that form electromagnetic (EM) waves. As previously mentioned, these
`waves do not require a medium to travel. This means that while they can travel through a medium like air or a
`wall,
`they can also travel
`through a vacuum like outer space. Electromagnetic waves are used a lot
`in
`communications systems,
`such as
`those used to talk to satellites
`in space.
`It should be noted that
`electromagnetic waves and electromagnetic radiation refer to the same phenomenon.
`
`Types of Waves
`
`Mechanical Wave Characteristics
`
`Characteristics of EM Radiation
`
`Wave Number and Impedance
`
`Attenuation of Waves
`
`Wave Interaction or Interference
`
`Refraction and Snell's Law
`
`Reflection and Transmission Coefficients
`
`Mode Conversion
`
`Wavelength and Defect Detection
`
`Signal-to-Noise Ratio
`
`Page 5 of 68
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`
`Glossary
`
`IAaa
`th and its
`cy, which is related to its
`One defining characteristic of an electromagnetic waveis its fre
`
`energy. These concepts will be discussed more in a following page. The electromagneti
`Im,
`is
`shown
`encompasses
`the
`full
`range
`of
`frequencies
`electromagnetic waves
`can
`have,
`VWhivalength (meters)
`The Electromagnetic Spectrum
`CMB
`human sight
`Radio
`Microwave
`Infrared
`Visible
`Ultraviolet
`103
`10°?
`10°5
`106
`10%
`
`Atomic Elements
`
`Introduction to Waves
`
`below.
`
`X-ray
`10°79
`
`Gamma Ray
`10°12
`
`Mechanical Wave Characteristics
`
`Frequency (Hz)
`
`Characteristics of EM Radiation
`
`104
`
`108
`
`4072
`
`4015
`
`4076
`
`1018
`
`4020
`
`Wave Number and Impedance
`
`Attenuation of Waves
`
`WaveInteraction or Interference
`
`Refraction and Snell's Law
`
`Reflection and Transmission Coefficients
`
`Mode Conversion
`
`Wavelength and Defect Detection
`
`Signal-to-Noise Ratio
`
`Review:
`
`1. The three main types of mechanical waves are transverse, mechanical, and surface.
`2. Mechanical waves only transport energy as they travel. They do not transport matter.
`3. Electromagnetic waves are formed from linked changing electric and magnetic fields.
`
`Page 6 of 68
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`About
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`Glossary
`
`
`
`Atomic Elements
`
`Physics of Nondestructive Evaluation > Waves > Mechanical Wave Characteristics
`
`Introduction to Waves
`
`Types of Waves
`
`Mechanical Wave Characteristics
`
`Characteristics of EM Radiation
`
`Wave Number and Impedance
`
`Attenuation of Waves
`
`Wave Interaction or Interference
`
`Refraction and Snell's Law
`
`Reflection and Transmission Coefficients
`
`Mode Conversion
`
`Wavelength and Defect Detection
`
`Signal-to-Noise Ratio
`
`Mechanical Wave Characteristics
`
`After reading this section you will be able to do the following:
`* Define periodic wave parameterslike w ngthand period.
`e Visualize periodic transverse waves and periodic longitudinal waves.
`
`When the disturbance causing the transferse or longitudinal waveis repetitive (periodic), a periodic wave occurs.
`
`Periodic Transverse Wave
`
`When periodic waves have simple ha ic motion they can also be called sinusoidal waves and any periodic
`wave can be described by a combination of sinusoidal waves.
`
`As a sinusoidal wave travels each particle undergoes simple harmonic motion with the same frequency, but does
`
`not travel with the wave. It is important to distinguish between the parti ion and the wave motion since
`waves transport energy, but not matter. The figure belowillustrates a traveling periodic transverse wave and
`points out different features of the wave like itswavelength and amplitude.
`
`elevation
`
`crest
`
`amplitude
`
`Page 7 of 68
`
`

`

`
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`
`
`
`elevation
`
`crest
`
`amplitude
`
`
`
`=>
`
`
`
`
`
`x
`>
`
`Motionof
`wees trough
`the wave
`
`
`wavelength
`
`The wavelength (represented by the Greek letter lambda) of a periodic wave is the physical distance between
`twa same points in the pattern (e.g., the distance between two crests or the distance between two troughs). Since
`wavelength is a distanceit is measured in distance units, typically meters. As a wave travels at a constant speed it
`advances 1 wavelengthin the time of 1 period. The period of a wave, which is denoted by T,
`is measured in
`seconds and the wave speed, which is the speed the wave travels at in its medium denoted by v, is measured in
`meters per second. Frequency is the number of wave cycles that will pass a point in 1 second. It is measured in
`the unit Hertz (Hz).
`
`Af
`
`=T
`
`A: wavelength
`
`Periodic Longitudinal Wave
`
`Atomic Elements
`
`Introduction to Waves
`
`Mechanical Wave Characteristics
`
`Characteristics of EM Radiation
`
`Wave Number and Impedance
`
`Attenuation of Waves
`
`WaveInteraction orInterference
`
`Refraction and Snell's Law
`
`Reflection and Transmission Coefficients
`
`Mode Conversion
`
`Wavelength and Defect Detection
`
`Signal-to-Noise Ratio
`
`Page 8 of 68
`
`

`

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`IOWA STATE UNIVERSITY
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`Glossary
`
` ) or pressure.
`ese regions are called
`compression an
`rarefaction, respectively, and are shownin thefigure below.Just like with a transverse periodic wave, a periodic
`Atomic Elements
` al wave has a wavelengththatis the physical distance between two same points in the pattern.In this
`case the wavelength can be defined as the distance between two compressionregions or two rarefaction
`regions.
`
`——_ direction of propagation
`
`Introduction to Waves
`
`Mechanical Wave Characteristics
`
`Characteristics of EM Radiation
`
`Wave Number and Impedance
`
`Attenuation of Waves
`
`WaveInteraction or Interference
`
`Refraction and Snell's Law
`
`Reflection and Transmission Coefficients
`
`Mode Conversion
`
`Wavelength and Defect Detection
`
`Signal-to-Noise Ratio
`
`ITTY
`
`
`
`
`
`
`
`
`
`
`
`TTTTTTTTETTTTETTT TTT
`
`
`
`
`
`ILTYTTTTLTTTTETAT TTT TTT
`‘iii
`MT VET MET
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`.. Compression Rarefaction Compression Rarefaction...
`
`Review:
`
`1. When periodic waves have simple harmonic motion they can also be called sinusoidal waves.
`2. Frequencyis the inverse of the wave period andvice versa.
`3, The wave speedis related to the wavelength and frequency of the wave.
`
`Page 9 of 68
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`Glossary
`
`
`Physics of Nondestructive Evaluation > Waves > Characteristics of EM Racliation
`
`Atomic Elements
`
`Characteristics of EM Radiation
`
`Introduction to Waves
`
`Mechanical Wave Characteristics
`
`Characteristics of EM Radiation
`
`Wave Numberand Impedance
`
`Attenuation of Waves
`
`Wave Interaction or Interference
`
`Refraction and Snell's Law
`
`Reflection and Transmission Coefficients
`
`Mode Conversion
`
`Wavelength and Defect Detection
`
`Signal-to-Noise Ratio
`
`After reading this section you will be able to do the following:
`* Explain what the electromagneticspectrumis and hew scientists use tt.
`
`So far we have learned about the atom, electricity, and magnetism,and have touched on electromagnetism. Two
`key pores ssto remember about the characteristics of EM radiation(e.g., microwaves, X-rays, and gammarays) are
`r, they are electromagnetic wave forms possessing no charge and no mass,
`and they canbe characterized by frequency, wavelength, and velocity. Let's take a closer look at
`the
`characteristics of these wave forms so that we may better understand the nature of them.
`
`Electromagnetic (EM) Waves
`X- and gamma rays are part of what scientists refer to as the electromagnetic (EM) spectrum. They are
`waveforms that are part of a family in which some ofthe relatives are very familiar to us, such as light rays,
`infrared heat rays, and radio waves. However, X- and gamma rays cannot been seen, felt, or heard.
`In other
`words, our normal senses cannot detect them. Since X- and gamma rays have no mass and noelectrical charge,
`they are not influenced by electrical and magnetic fields and will travel in straight lines.
`
`Radiation possesses a dual character. Acting somewhatlike a particle at times and like a wave at other times. The
`name that has been given to the small “packets” of energy with these characteristics is "photon."It is said that
`
`Page 10 of 68
`
`

`

`haracterradiation.xhtmil
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`IOWA STATE UNIVERSITY
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`
`Atomic Elements
`
`been described in terms of a stream of photons (massless particles) each traveling in a wave-like pattern and
`moving at the speed oflight.
`
`This diagram shows the electromagnetic spectrum. Notice the changes in wavelengths of the various wave forms.
`
`Introduction to Waves
`
`Mechanical Wave Characteristics
`
`Characteristics of EM Radiation
`
`Wave Number and Impedance
`
`Attenuation of Waves
`
`Wave Interaction or Interference
`
`Refraction and Snell's Law
`
`Reflection and Transmission Coefficients
`
`Mode Conversion
`
`Wavelength and Defect Detection
`
`Signal-to-Noise Ratio
`
`Ultraviolet
`Thermal
`__ Radiation Radiation
`
`
`
`
`
`
`
`‘Visible
`“X-RaysGammaRays
`~ Radio Waves Microwaves
`Light
`
`Wavelengity meters}
`Radio
`Microwave
`103
`402
`
`The Electromagnetic Spectrum
`Infrared
`Visible
`Ultraviolet
`105
`410%
`108
`
`X-ray
`40-10
`
`Gamma Ray
`4072
`
`Frequency (Hz)
`
`104
`
`108
`
`1012
`
`4018
`
`1016
`
`4078
`
`1020
`
`Every point across the spectrumrepresents a wave form of differing wavelength. It should be noted that the lines
`between the groupings are not precise, and that each group phasesinto the next.
`
`Wave forms may be graphically represented as following:
`
`Page 11 of 68
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`

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`IOWA STATE UNIVERSITY
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`About
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`
`Glossary
`
`
`
`Atomic Elements
`
`Introduction ta Waves
`
`Mechanical Wave Characteristics
`
`Characteristics of EM Radiation
`
`Wave Number and Impedance
`
`Attenuation of Waves
`
`WaveInteraction or Interference
`
`Refraction and Snell's Law
`
`Reflection and Transmission Coefficients
`
`Mode Conversion
`
`Wavelength and Defect Detection
`
`Signal-to-Noise Ratio
`
`x-position
`
`
`y-position
`
`Wavelength
`
`Wavelength
`Just as with mechanical waves on the previous page, the distance between the peaks or the distance between the
`troughs of the wavesis the length of one wave, or more precisely the wavelength. Note that the electromagnetic
`waves vary in length from one end of the spectrumto the other. Some radio waves may be several miles longs,
`while X and gamma rays are measured in angstroms, and fractions of angstrom. An angstrom unit is equivalent
`to 0.00000001 centimeters (100 millionths cm).
`If you want to compare this to an inch, a centimeter is equal to
`0.394 inch. You can see that the wavelength as measured in angstrom units is extremely small.
`
`Frequency
`Another term used to describe a waveis frequency. Since waves are moving, we define frequency as the number
`
`Page 12 of 68
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`Atomic Elements
`
`
`of wave cycles pass a point in one second. So one cycle per second equals one Hertz. . The frequencyof a waveis
`indirectly proportional to the wavelengthof the radiation. This means that as one value goes up, the other goes
`down. You can see that as the wavelengthincreases, it takes longer for one complete wave to pass a point and,
`therefore, the frequency goes down.
`
`Introduction to Waves
`
`Mechanical Wave Characteristics
`
`Characteristics of EM Radiation
`
`Wave Number and Impedance
`
`Attenuation of Waves
`
`Wave Interaction or Interference
`
`Refraction and Snell's Law
`
`Reflection and Transmission Coefficients
`
`Mode Conversion
`
`Wavelength and Defect Detection
`
`Signal-to-Noise Ratio
`
`Frequencyvs. Velocity
`Don't confuse the frequency of a wave with the speed oftravel or velocity of a wave; they are not the same or
`even related. X and gamma rays and all other members of the electromagnetic spectrum, travel at the same
`speed, 186,000 miles per second (3x10*8 meters per second). This speed is known as the speed oflight, or
`actually the speed of electromagnetic radiation. Frequency describes how many complete wave cycles go flying
`by in one second.
`
`Let's pause for a quick check of understanding
`
`Now would be a good time to apply what we have learned about the characteristics of electromagnetic waves.
`Lets look at some specific examples and see if we understand.
`
`Here are two wave forms (wave A, and waveB) with different wavelength. Which one has the greater wavelength
`and which one has the greater frequency? If wave A has a frequency of 1 Hertz, whatis the frequencyof wave B?
`
`Wavelorm A
`
`
`Page 13 of 68
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`IOWA STATE UNIVERSITY
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`Glossary
`
`ra
`
`Atomic Elements
`
`Wavelonn A
`
`Introduction to Waves
`
`Characteristics of EM Radiation Plane Waves
`
`Mechanical Wave Characteristics
`
`Wave Number and Impedance
`Abanister cl ike
`WaveInteraction or Interference
`Refraction and Snell's Law
`
`Reflection and Transmission Coefficients
`
`Mode Conversion
`
`Wavelength and Defect Detection
`Rati
`land heels Hate
`
`You should have concluded that Wave A has a longer wavelength; therefore it will have a lower frequency than
`Wave B. Remember wenieiorie) and frequencyis inversely proportional; meaning that as one increases, the other
`decreases by a proportional amount. You should have also concluded that Wave B has a frequency of 2 Hertz. For
`every complete cycle of Wave A, Wave B completes2 cycles.
`
`Importance of Wavelength and Frequency
`Not only are the wavelength and frequency of the wavelinked, but they are also linked to the amount of energy
`of the wave.
`If two waves have that same amplitude then they each have the same amount of energy in one
`amples
`complete a cycle. Since waves with short wavelengths complete more cycles per second, you can see that they
`can transfer more energy than waves with longer wavelengths can in the same amount of time. Therefore,
`
`?
`
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`Glossary
`
`
`spectrum, you will note that the shorter wavelength rays posses more energy.
`
`Atomic Elements
`
`Introduction to Waves
`
`Mechanical Wave Characteristics
`
`Characteristics of EM Radiation
`
`Wave Number and Impedance
`
`Attenuation of Waves
`
`WaveInteraction or Interference
`
`Refraction and Snell's Law
`
`Reflection and Transmission Coefficients
`
`Mode Conversion
`
`Wavelength and Defect Detection
`
`Signal-to-Noise Ratio
`
`Importance of the Energy of Radiation
`It
`is important to know the energy of the radiation because the energy controls the penetrating power of the
`radiation. Higher energy radiation can penetrate more amounts of material and harder to penetrate materials.
`This is important for a doctor taking x-rays so that there is not too much energy which can harm their patient. In
`NDTit's important so that we can properly penetrate the material or structure we wantto inspect.
`
`Measuring Radiation
`lt
`is common practice to measure x and gamma rays in one of two units, which are thousand electronvolts
`(Kev), or million electron valts (Mev). What is an electron valt, you might ask? An electron volt is an amount of
`energy equal to the energy gained by one electron when it is accelerated by one volt. For example,
`if one
`electron was accelerated by a potential of 10 thousand volts (10 Kv), then the electron would have an energy of
`10 thousand electronvolts (10 Kev). If all of the accelerating energy were converted to electromagnetic energy,it
`would result in a 10 Kev x-ray.
`
`Review:
`
`1. Radiationis an electromagnetic (EM) wave that has no charge and no mass.
`2. EM radiation can be characterized by frequency, wavelength, and energy.
`
`Page 15 of 68
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`
`Physics of Nondestructive Evaluation > Waves > Properties of Plane Waves
`
`Atomic Elements
`
`Properties of Plane Waves
`
`After reading this section you will be able to do the following:
`e Describe properties of waves.
`¢ Explain what a planewaveis.
`
`Introduction to Waves
`
`Mechanical Wave Characteristics
`
`Characteristics of EM Radiation
`
`Wave Numberand Impedance
`
`Attenuation of Waves
`
`WaveInteraction or Interference
`
`Refraction and Snell's Law
`
`Reflection and Transmission Coefficients
`
`Mode Conversion
`
`Wavelength and Defect Detection
`
`Signal-to-Noise Ratio
`
`Wavelength, Frequency and Velocity
`Velocity: The distance covered by the
`wavein unit time.
`
`for mechanical waves among the
`As previously discussed,
`properties of waves propagating in isotropic solid materials are
`wavelength, frequency, and velocity. The wavelengthis directly
`proportional
`to the velocity
`of
`the wave
`and_
`inversely
`proportional to the frequency of the wave. This relationship is
`shown by the following equation.
`
`Velocity(v)
`Wavelength(A) = Frequency(f)
`
`wave.
`
`Frequency: The number of complete
`disturbances (cycles) in unit time.
`Usually expressed in Hertz.
`
`Wavelength: The distance between
`successive points of equal phase ina
`
`
`The applet below showsa longi | and transverse wave. The
`
`direction of wave propagation is
`from left to right and the
`movementofthe lines indicate the direction of particle oscillation. The equation relating ultrasonic wavelength,
`frequency, and propagationvelocityis included at the bottom of the applet in a reorganized form. The values for
`
`Page 16 of 68
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`Glossary
`
`wave. Note that the frequencyvalue must be kept between 0.1 to 1 MHz (one million cycles per second) and the
`wave velocity must be between 0.1 and 0.7 cm/us.
`
`Longitude Wave
`
`‘nn
`
`
`
`
`
`IOWA STATE UNIVERSITY
`Center for Nondestructive Evaluation
`
`Atomic Elements
`
`Introduction to Waves
`
`Mechanical Wave Characteristics
`
`Characteristics of EM Radiation
`
`Wave Number and Impedance
`
`Attenuation of Waves
`
`Wave Interaction or Interference
`
`Refraction and Snell's Law
`
`
`
`
`
`
`
`
`
`Wavelength: 2.000 cm
`
`Velocity: 0.600 cm/us
`
`Frequency: 0.300 mHz
`
`Time acceleration: 1x
`
`
`
`
`
`
`
`
`
`SEwavelength * frequency = velocity
`
`Wavelenglt: sy
`Frequency: ==
`Reflection and Transmission Coefficients
`Velocity:SSESD
`Mode Conversion
`
`Wavelength and Defect Detection
`
`Signal-to-Noise Ratio
`
`*
`
`Changethe settings for the applet and observe how the waveforms change.
`
`Page 17 of 68
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`Glossary
`
`As can be noted by the equation, a change in frequency will result in a change in wavelength. Change the
`frequency in the applet and view the resultant wavelength. At a frequency of .2 and a material velocity of 0.585
`(longitudinal wavein steel) note the resulting wa
`
`gth. Adjust the material velocity to 0.480 (longitudinal wave
`in cast iron) and note the resulting wavelength. Increase the frequencyto 0.8 and note the shortened wavelength
`in each material.
`
`In ultrasonic testing, the shorter wavelengthresulting from an increase in frequencywill usually provide for the
`detection of smaller discontinuities. This will be discussed morein following sections.
`
`Plane Wave
`
`Plane wavesare a special case of waves where a physical quantity, such as phase, is constant over a plane thatis
`perpendicular to the direction of wave travel. There can be mechanical (both transverseand longitudinal) as well
`as EM plane waves.It should be noted that EM plane wavesare only tra
`». A visualization of a pl
`/eis
`
`shownbelow.Just like periodic waves, plane waves have a wavelength
`cy, and wave speed.
`
`IOWA STATE UNIVERSITY
`Center for Nondestructive Evaluation
`
`Atomic Elements
`
`Introduction to Waves
`
`Mechanical Wave Characteristics
`
`Characteristics of EM Radiation
`
`Wave Number and Impedance
`
`Attenuation of Waves
`
`Wave Interaction or Interference
`
`Refraction and Snell's Law
`
`Reflection and Transmission Coefficients
`
`Mode Conversion
`
`Wavelength and Defect Detection
`
`Signal-to-Noise Ratio
`
`
`
`|
`
`
`
`Phase Velocity
`
`Page 18 of 68
`
`

`

`iy)
`
`errssecsRelceiia
`
`IOWA STATE UNIVERSITY
`Center for Nondestructive Evaluation
`
`Atomic Elements
`
`Introduction to Waves
`
`Mechanical Wave Characteristics
`
`Characteristics of EM Radiation
`
`Wave Number and Impedance
`
`Attenuation of Waves
`
`Wave Interaction or Interference
`
`Refraction and Snell's Law
`
`Reflection and Transmission Coefficients
`
`Made Conversion
`
`Wavelength and Defect Detection
`
`Signal-to-Noise Ratio
`
`About
`
`Careers
`
`Physics
`
`Techniques
`
`NDEEngineering
`
`Glossary
`
`When EM wavesradiate from a source, such as an antenna, they radiate as spherical waves (see the figure below).
`As the waves movefarther from the source, their energy gets spread out over a bigger spherical surface area. At
`big distances, which wecall the far-field, a spherical wave front can be approximated as a uniform plane wave
`over a defined area.
`
`cet etEltlen]
`
`Decreasing concentration of
`
`Review:
`
`1. Plane waves are a special case of waves where a physical quantity, such as phase, is constant over a plane
`that is perpendicular to the direction of wavetravel.
`2. Just like periodic waves, plane waves have a wavelength, frequency, and wave speed.
`
`Page 19 of 68
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`

`

`Ss nde-ed.org/Physics
`
`IOWA STATE UNIVERSITY
`Center for Nondestructive Evaluation
`
`Atomic Elements
`
`Introduction to Waves
`
`Mechanical Wave Characteristics
`
`Characteristics of EM Radiation
`
`Wave Number and Impedance
`
`Attenuation of Waves
`
`WaveInteraction or Interference
`
`Refraction and Snell's Law
`
`Reflection and Transmission Coefficients
`
`Mode Conversion
`
`Wavelength and Defect Detection
`
`Signal-to-Noise Ratio
`
`About
`
`Careers
`
`Physics
`
`Techniques
`
`NDEEngineering
`
`Glossary
`
`
`Physics of Nondestructive Evaluation > Waves > Wave Number and Impedance
`
`Wave Number and Impedance
`
`After reading this section you will be able to do the following:
`© Describe different wave parameters that are related to the material the waveis in.
`
`Refractive Index
`
`Refractiveindexis a material property that determines how much light (or a wave at other frequencies) is bent
`and how much is reflected/transmitted when it travels from one material to another. The refractiveindexis
`related to the complex dielectric constant by the following equation:
`
`As a reminder:
`
`n= Ver
`
`€ = 6€9
`
`et
`
`WaveSpeed and Speed ofLight
`In free space an EM wavestravels at the speed of light, which is approximately 3x10*8 m/s. In a material, an EM
`wave travels more slowly as a function of the dielectric constant of the material. In general, the wave speed(vp)is
`related to the permeability and the permittivity by the following equation:
`
`e
`
`1
`
`2 a=a
`
`Page 20 of 68
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`Glossary
`
`OSI—————————E—E—E—E—E—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_—_
`
`Atomic Elements
`
`Vy = a
`
`Whenthe waveis traveling in free space this equation works out such that v, equals the speedoflight. You can
`test this yourself by plugging in the following values:
`
`= we, = (40x 107-7) () = 4 X107H /m
`
`€ = €,€,
`
`= (8.854 x 10°1)(1) = 8.854 x 10°17F / m
`
`A
`I
`You should het clase i 3x10"5. m/s!
`
`Wavenumber
`The wavenumber,also called the propagation number or propagationconstant, can be thoughtof as a spatial
`frequency - how many wavelengths occur per unit distance. In free space it is the ratio of the angularfrequency
`to the speed oflight and in a materialit is related to the refractive index.
`
`Intrinsic Impedance
`The intrinsic impedanceis the impedance an EM wavefacesasit travels through a medium. When the medium
`is homogeneous and the EM waveis a transverse electromagnetic (TEM) wave, then the intrinsic impedanceis
`equal to the wave impedance. A TEM waveis simply a special case of EM waves wherethe linked electric fields
`and magnetic fields that make up the wave are orthogonal to each other and to the direction that the waveis
`traveling in. A visualization of a TEM waveis shown below,
`
`Introduction to Waves
`
`Mechanical Wave Characteristics
`
`Characteristics of EM Radiation
`
`Wave Number and Impedance
`Atannaton bf Waves
`
`Wave Interaction or Interference
`Refraction and Snell's Law
`Reflection and Transmission Coefficients
`Mada Conversion
`
`Wavelength and Defect Detection
`
`Signal-to-Noise Ratio
`
`i
`
`Page 21 of 68
`
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`IOWA STATE UNIVERSITY
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`Glossary
`
`
`
`k=
`
`Wavelength
`
`Electric
`
`field
`
`+r
`
`Atomic Elements
`
`Introduction to Waves
`
`Mechanical Wave Characteristics
`
`Characteristics of EM Radiation
`
`Wave Numberand Impedance
`
`Attenuation of Waves
`
`Wave Interaction or Interference
`
`Refraction and Snell's Law
`
`Reflection and Transmission Coefficients
`
`Mode Conversion
`
`Wavelength and Defect Detection
`
`Signal-to-Noise Ratio
`
`When the waveis traveling in a dielectric then the intrinsic impedance(represented by the Greek letter eta) can
`be described with the following equation:
`
`=fE7
`
`é€
`
`The equation above showsthatthe intrinsic impedanceis related to the permeability and the permittivity of the
`material.
`
`Review:
`
`1. Refractive index, wave speed, wave number,and intrinsic impedance are all interrelated quantities.
`2. Chaning the material properties changes how a wave will propagate t