Lesson 1: The Nature of a Sound WaveSound is a Mechanical Wave Sound is a Longitudinal Wave Sound is a Pressure Wave Lesson 2: Sound Properties and Their PerceptionPitch and Frequency The Speed of Sound Lesson 3: Behavior of Sound WavesInterference and Beats The Doppler Effect and Shock Waves Boundary Behavior Reflection, Refraction, and Diffraction Lesson 4: Resonance and Standing WavesNatural Frequency Forced Vibration Standing Wave Patterns Fundamental Frequency and Harmonics

Chapter 10 – Part I - Sound

The Nature of a Sound Wave- objectives

• Sound is a Mechanical Wave

• Sound is a Longitudinal Wave

• Sound is a Pressure Wave

Sound is a Mechanical Wave• A wave can be described as a disturbance that travels through a

medium, transporting energy from one location to another location. A wave is created by vibrating objects.

• Mechanical wave is a wave that propagates through a medium from one location to another.

• The medium is simply the material through which the disturbance is moving; it can be thought of as a series of interacting particles.

– The coil in a slinky

– The water particles in the ocean

– The people in the stadium

The tines of a tuning fork vibrates at a very high frequency. If the tuning fork that is being used corresponds to middle C on the piano keyboard, then the tines are vibrating at a frequency of 256 Hertz; that is, 256 vibrations per second.

• Some tuning forks are mounted on a sound box. In such instances, the vibrating tuning fork, being connected to the sound box, sets the sound box into vibrating motion. In turn, the sound box, being connected to the air inside of it, sets the air inside of the sound box into vibrating motion.

• As the tines of the tuning fork, the structure of the sound box, and the air inside of the sound box begin vibrating at the same frequency, a louder sound is produced. In fact, the more particles which can be made to vibrate, the louder or more amplified the sound.

Mechanical waves vs. electromagnetic waves

.• Electromagnetic waves are waves that have an electric and magnetic nature and are capable of traveling through a vacuum. Electromagnetic waves do not require a medium in order to transport their energy.

• Mechanical waves are waves that require a medium in order to transport their energy from one location to another. Because mechanical waves rely on particle interaction in order to transport their energy, they cannot travel through a vacuum.

The sound produced by the bell cannot be heard since sound cannot travel through a vacuum.

..\..\sound_en.jar

Check Your Understanding

1. A sound wave is different than a light wave in that a sound wave is

a. produced by an oscillating object and a light wave is not.

b. not capable of traveling through a vacuum.

c. not capable of diffracting and a light wave is.

d. capable of existing with a variety of frequencies and a light wave has a single frequency.

Sound as a Longitudinal Wave• Longitudinal waves are waves in which the motion of the

individual particles of the medium is in a direction that is parallel to the direction of energy transport.

http://einstein.byu.edu/~masong/HTMstuff/WaveTrans.html

Sound is a Pressure Wave• vibrating tuning fork is capable of creating such a longitudinal

wave. As the tines of the fork vibrate back and forth, they push on neighboring air particles. The forward motion of a tine pushes air molecules horizontally creates high-pressure area and the backward retraction of the tine creates a low-pressure area allowing the air particles to move back to the left.

• High pressure regions are known as compressions and low pressure regions rarefactions.

• The wavelength is commonly measured as the distance from one compression to the next adjacent compression.

• Since a sound wave consists of a repeating pattern of high pressure and low pressure regions moving through a medium, it is sometimes referred to as a pressure wave.

example• A sound wave is a pressure wave; regions of high

(compressions) and low pressure (rarefactions) are established as the result of the vibrations of the sound source. These compressions and rarefactions result because sound

1. is more dense than air and thus has more inertia, causing the bunching up of sound.

2. waves have a speed which is dependent only upon the properties of the medium.

3. is like all waves; it is able to bend into the regions of space behind obstacles.

4. is able to reflect off fixed ends and interfere with incident waves

5. vibrates longitudinally; the longitudinal movement of air produces pressure fluctuations.

example

• As a sound wave travels through air, there is a net transfer of

1. energy, only

2. mass, only

3. both mass and energy

4. neither mass nor energy

example• The diagram shows a tuning fork vibrating in air.  The

dots represent air molecules as the sound wave moves towards the right.  Which diagram below best represents the direction of motion of the air molecules?

A B C D

example• An electric bell connected to a battery is sealed inside a

large jar. What happens as the air is removed from the jar?

1. The electric circuit stops working because electromagnetic radiation cannot travel through a vacuum.

2. The bell's pitch decreases because the frequency of the sound waves is lower in a vacuum than in air.

3. The bell's loudness increases because of decreased air resistance.

4. The bell's loudness decreases because sound waves cannot travel through a vacuum.

example• A student plucks a guitar string and the

vibrations produce a sound wave with a frequency of 650 hertz. The sound wave produced can best be described as a

1. transverse wave of constant amplitude

2. longitudinal wave of constant frequency

3. mechanical wave of varying frequency

4. electromagnetic wave of varying wavelengths

Lesson 2: Sound Properties and Their Perception

1. Relate Pitch and Frequency

2. Relate Intensity and the amplitude

3. Explain the factors affecting The Speed of Sound

Pitch and Frequency• A sound wave, like any other wave, is introduced into a medium

by a vibrating object.

• Regardless of what vibrating object is creating the sound wave, the particles of the medium through which the sound moves is vibrating in a back and forth motion at a frequency same as its source.

• The frequency of a wave refers to how often the particles of the medium vibrate when a wave passes through the medium. If a particle of air undergoes 1000 longitudinal vibrations in 2 seconds, then the frequency of the wave would be ____________ vibrations per second. A commonly used unit for frequency is the Hertz (abbreviated Hz), where 1 Hertz = 1 vibration/second

• The period of the sound wave is the time between successive high pressure points. The frequency is the reciprocal of the period.

• The sensation of a frequency of sound is commonly referred to as the pitch. A high pitch sound corresponds to a high frequency sound wave and a low pitch sound corresponds to a low frequency sound wave.

• The ears of a human (and other animals) are sensitive detectors capable of detecting the fluctuations in air pressure which impinge upon the eardrum. The human ear is capable of detecting sound waves with a wide range of frequencies, ranging between approximately 20 Hz to 20 000 Hz. Any sound with a frequency below the audible range of hearing (i.e., less than 20 Hz) is known as an infrasound and any sound with a frequency above the audible range of hearing (i.e., more than 20 000 Hz) is known as an ultrasound.

Ear drum

The energy of a sound wave is its intensity (loudness)

• The amount of energy which is transferred to the medium is dependent upon the amplitude of the wave.

• For example, if more energy is put into the plucking of the string (that is, more work is done to displace the string a greater amount from its rest position), then the string vibrates with a greater amplitude.

• The amount of energy which is transported past a given area of the medium per unit of time is known as the intensity (loudness) of the sound wave. The greater the amplitude of vibrations of the particles of the medium, the more intense that sound wave is.

As a sound wave carries its energy through a medium, the intensity of the sound wave ____________ with increasing distance from the source.

The mathematical relationship between intensity and distance is sometimes referred to as an inverse square relationship.

The scale for measuring intensity is the decibel scale.

decreases

example• A system consists of an oscillator and a speaker

that emits a 1,000.-hertz sound wave. A microphone detects the sound wave 1.00 meter from the speaker. The microphone is moved to a new fixed location 0.50 meter in front of the speaker. Compared to the sound waves detected at the 1.00-meter position, the sound waves detected at the 0.50-meter position have a different

1. wave speed2. frequency3. wavelength4. amplitude

example

• Light is to brightness as sound is to

1. color

2. loudness

3. period

4. speed

The Speed of Sound• Like any wave, the speed of a sound wave refers to how fast the

disturbance is passed from particle to particle. While frequency refers to the number of vibrations which an individual particle makes per unit of time. Speed and frequency are different quantities.

• speed = distance/time

• The faster a sound wave travels, the more distance it will cover in the same period of time.

Factors Affecting Wave Speed • The speed of any wave depends upon the properties of

the medium through which the wave is traveling. • Typically, sound wave travels faster in denser

materials.

• vsolids > vliquids > vgases

Sound waves travel faster in solids than they do in liquids than they do in gases.

example• What occurs when sound passes from air into

water?

1. Its speed decreases, its wavelength becomes smaller, and its frequency remains the same.

2. Its speed decreases, its wavelength becomes smaller, and its frequency increases.

3. Its speed increases, its wavelength becomes larger, and its frequency remains the same.

4. Its speed increases, its wavelength becomes larger, and its frequency decreases.

Sound in air• The speed of a sound wave in air depends upon the properties of

the air, namely the temperature and the pressure. • The speed of sound wave at STP is 331 m/s• At normal atmospheric pressure, the temperature dependence of

the speed of a sound wave through air is approximated by the following equation:

v = 331 m/s + (0.6 m/s/oC)•T– where T is the temperature of the air in degrees Celsius.

• Using this equation, we can determine the speed of a sound wave in air at a temperature of 20 degrees Celsius:

Using Wave Speed to Determine Distances

• At normal atmospheric pressure and a temperature of 20oC, a sound wave will travel at approximately 343 m/s;

• The speed of a sound wave is slow in comparison to the speed of a light wave. Light travels through air at a speed of approximately 3 x 108 m/s;

• The time delay between the arrival of the light wave (lightning) and the arrival of the sound wave (thunder) allows a person to approximate his/her distance from the storm location.

• For instance if the thunder is heard 5 seconds after the lightning is seen, then sound has traveled a distance of d = v • t = 345 m/s • 5 s = 1715 m, which means the storm is about one mile away.

Every 5 seconds is about a mile.

• Another phenomenon related to the perception of time delays between two events is an echo.

• For instance if an echo is heard 1.40 seconds after making the holler, then the distance to the canyon wall can be found as follows:

echolocation

The Wave Equation Revisited• Like any wave, a sound wave has a speed which is

mathematically related to the frequency and the wavelength of the wave.

• Speed = Wavelength • Frequency v = f • λ• Even though wave speed is calculated using the frequency and

the wavelength, the wave speed is not dependent upon these quantities. – An alteration in wavelength does not affect (i.e., change)

wave speed. Rather, an alteration in wavelength affects the frequency in an inverse manner.

– A doubling of the wavelength results in a halving of the frequency; yet the wave speed is not changed.

– The speed of a sound wave depends on the properties of the medium through which it moves and the only way to change the speed is to change the properties of the medium.

example• A sound wave is produced by a musical instrument for 0.50

second. If the frequency of the wave is 360 hertz, how many complete waves are produced in that time period?

Check Your Understanding1. An automatic focus camera is able to focus on objects by

use of an ultrasonic sound wave. The camera sends out sound waves that reflect off distant objects and return to the camera. A sensor detects the time it takes for the waves to return and then determines the distance an object is from the camera. If a sound wave (speed = 340 m/s) returns to the camera 0.150 seconds after leaving the camera, how far away is the object?

2. On a hot summer day, a pesky little mosquito produced its warning sound near your ear. The sound is produced by the beating of its wings at a rate of about 600 wing beats per second.

a. What is the frequency in Hertz of the sound wave?

b. Assuming the sound wave moves with a velocity of 350 m/s, what is the wavelength of the wave?

3. Doubling the frequency of a wave source doubles the speed of the waves.a. True b. False 

 4. Playing middle C on the piano keyboard produces a sound

with a frequency of 256 Hz. Assuming the speed of sound in air is 345 m/s, determine the wavelength of the sound corresponding to the note of middle C.

5. Most people can detect frequencies as high as 20 000 Hz. Assuming the speed of sound in air is 345 m/s, determine the wavelength of the sound corresponding to this upper range of audible hearing.

 

6. An elephant produces a 10 Hz sound wave. Assuming the speed of sound in air is 345 m/s, determine the wavelength of this infrasonic sound wave.

Behavior of Sound Waves objectives

1. Understand Interference and Beats

2. Explain The Doppler Effect and Shock Waves

3. Recognize Boundary Behavior - Reflection, Refraction, and Diffraction

Interference and Beats• Wave interference is the phenomenon which occurs when two

waves meet while traveling along the same medium. The interference of waves causes the medium to take on a shape which results from the net effect of the two individual waves upon the particles of the medium.

• Constructive interference occurs if two waves are moving in the same direction at the same time meet with each other. Constructive interference result a ________ displacement.

• destructive interference occurs if two waves are moving in the opposite directions at the same time when they meet. Destructive interference result in a __________ displacement.

larger

smaller

• Sound is a pressure wave which consists of compressions and _______________, or a high pressure region and a low pressure region.

• The interference of sound waves causes the particles of the medium to behave in a manner that reflects the net effect of the two individual waves upon the particles.

• If a compression (high pressure) of one wave meets up with a compression (high pressure) of a second wave at the same location in the medium, then the net effect is that that particular location will experience an even greater pressure. This is a form of ______________ interference.

• If two rarefactions (two low pressure disturbances) from two different sound waves meet up at the same location, then the net effect is that that particular location will experience an even lower pressure. This is also an example of ______________ interference.

rarefactions

constructive

constructive

• Now if a particular location along the medium repeatedly experiences the interference of two compressions followed up by the interference of two rarefactions, then the two sound waves will continually reinforce each other and produce a very loud sound. The loudness of the sound is the result of the particles at that location of the medium undergoing oscillations from very high to very low pressures. locations along the medium where constructive interference continually occurs are known as __________.anti-nodes

• Now if two sound waves interfere at a given location in such a way that the compression of one wave meets up with the rarefaction of a second wave, ___________ interference results. As a result, the particles would remain at their rest position as though there wasn't even a disturbance passing through them. The two sound waves will continually cancel each other and no sound is heard. Locations along the medium where destructive interference continually occurs are known as nodes.

destructive

• Destructive interference of sound waves becomes an important issue in the design of concert halls and auditoriums. The rooms must be designed in such as way as to reduce the amount of destructive interference. One means of reducing the severity of destructive interference is by the design of walls, ceilings, and baffles that serve to absorb sound rather than reflect it.

• The destructive interference of sound waves can also be used advantageously in noise reduction systems. Ear phones have been produced which can be used by factory and construction workers to reduce the noise levels on their jobs. Such ear phones capture sound from the environment and use computer technology to produce a second sound wave which one-half cycle out of phase. The combination of these two sound waves within the headset will result in destructive interference and thus reduce a worker's exposure to loud noise.

Musical Beats• Interference of sound waves has widespread applications in the

world of music. In fact, the major distinction between music and noise is that noise consists of a mixture of frequencies whose mathematical relationship to one another is not readily noticeable. On the other hand, music consists of a mixture of frequencies which have a clear mathematical relationship between them.

•Note: the diagrams on this page represent a sound wave by a sine wave. Because the variations in pressure with time take on the pattern of a sine wave. Sound is not a transverse wave, but rather a longitudinal wave.

• The beat frequency refers to the rate at which the volume is heard to be oscillating from high to low volume. For example, if two complete cycles of high and low volumes are heard every second, the beat frequency is 2 Hz. The beat frequency is always equal to the difference in frequency of the two notes which interfere to produce the beats. So if two sound waves with frequencies of 256 Hz and 254 Hz are played simultaneously, a beat frequency of 2 Hz will be detected.

Doppler effect• We are most familiar with the Doppler effect because of our

experiences with sound waves. Perhaps you recall an instance in which a police car or emergency vehicle was traveling towards you on the highway. As the car approached with its siren blasting, the pitch of the siren sound (a measure of the siren's frequency) was _______; and then suddenly after the car passed by, the pitch of the siren sound was ______. That was the Doppler effect - a shift in the apparent frequency for a sound wave produced by a moving source.

high

low

Explaining the Doppler Effect• The Doppler effect is observed because the distance between the

source of sound and the observer is changing. • If the source and the observer are approaching each other, then

the distance is decreasing and the waves is compressed into the smaller distance. The observer perceives sound waves reaching him or her at a more frequent rate (_______ pitch).

• If the source and the observer are moving apart, then the distance is increasing. the waves can be spread apart; the observer perceives sound waves reaching him or her at a less frequent rate (____pitch).

• It is important to note that the effect does not result because of an actual change in the frequency of the source. The source puts out the same frequency; the observer only perceives a different frequency because of the relative motion between them.

high

low

•The Doppler effect is a shift in the observed frequency and not a shift in the actual frequency at which the source vibrates.

Shock Waves and Sonic Booms• If a moving source of sound moves at the same

speed as sound or faster than sound, then shock waves will be produced.

Check Your Understanding1. Suppose you are standing on the passenger-loading platform of

the commuter railway line. As the commuter train approaches the station, what pitch or changes in pitch will you perceive as the train approaches you on the loading platform?

Boundary Behavior• As a sound wave travels through a medium, it will often reach

the end of the medium and encounter another medium through which it could travel. The behavior of a wave upon reaching the end of a medium is referred to as boundary behavior.

• There are essentially three possible behaviors which a wave could exhibit at a boundary:– ____________ (the bouncing off of the boundary), – _____________ (the bending around the obstacle without

crossing over the boundary), – ________________ (the crossing of the boundary into the

new material or obstacle), – ________________ (occurs along with transmission and is

characterized by the subsequent change in speed and direction).

reflection

diffraction

transmission

refraction

Reflection - echo or a reverberation. • A reverberation often occurs in a small room

with height, width, and length dimensions of approximately 17 meters or less. The reflected sound wave has a very short delay, it seems to the person that the sound is prolonged. You might observe reverberations when talking in an empty room, when honking the horn while driving through a highway tunnel or underpass, or when singing in the shower.

• Echoes occur when a reflected sound is perceived as a second sound rather than the prolonging of the first sound.

Diffraction of Sound Waves• Diffraction involves a change in direction of waves as

they pass through an opening or around a barrier in their path.

• Diffraction of sound waves is commonly observed; we notice sound diffracting around corners or through door openings, allowing us to hear others who are speaking to us from adjacent rooms.

Refraction of Sound Waves• Refraction of waves involves a change in the direction of

waves as they pass from one medium to another. Refraction, or bending of the path of the waves, is accompanied by a change in speed and wavelength of the waves. So if the medium (and its properties) are changed, the speed of the waves are changed. Thus, waves passing from one medium to another will undergo refraction.

• Refraction of sound waves is most evident in situations in which the sound wave passes through a medium with gradually varying properties.

example

• A stationary research ship uses sonar to send a 1.18 × 103-hertz sound wave down through the ocean water. The reflected sound wave from the flat ocean bottom 324 meters below the ship is detected 0.425 second after it was sent from the ship. Calculate the wavelength of the sound wave in the ocean water.

Resonance

• Natural Frequency

• Forced Vibration

• Standing wave pattern

Natural Frequency• Nearly all objects, when hit or struck or plucked or

strummed or somehow disturbed, will vibrate. The frequency or frequencies at which an object tends to vibrate with when disturbed is known as the natural frequency of the object.

• If you were to take a guitar string and pluck it, you would hear a sound;

• On the other hand, if the string is attached to the sound box of the guitar and you pluck it, the sound produced would be much louder. This is because the vibrating string is capable of forcing the sound box into vibrating at that same natural frequency. The sound box in turn forces air particles inside the box into vibrating motion at the same natural frequency as the string. The string, guitar, and enclosed air begins vibrating and forces surrounding air particles into vibrational motion.

• The tendency of one object to force another adjoining or interconnected object into vibrational motion is referred to as a forced vibration. The forced vibration causes an increase in the amplitude and thus loudness of the sound.

Forced vibration

The result of resonance is always a large vibration. This is often demonstrated with an odd-looking mechanical system resembling an inverted pendulum.

When the red bob is disturbed, it begins vibrating at its natural frequency. This in turn forces the attached bar to vibrate at the same frequency; and this forces the other attached red bob into vibrating at the same natural frequency.

Resonance• This is resonance - one bob vibrating at a given frequency

forcing a second object with the same natural frequency into vibrational motion.

• While the green and the blue bobs were disturbed by the vibrations transmitted through the metal bar, only the red bob would resonate. This is because the frequency of the first red bob is tuned to the frequency of the second red bob; they share the same natural frequency. The result is that the second red bob begins vibrating with a huge amplitude.

• Resonance - when one object vibrating at the same natural frequency of a second object forces that second object into vibrational motion.

• Condition for resonance:

1. when the frequency of the periodic force equals to the natural frequency of the object it applied to.

2. The amplitude of the original wave is big enough.

Examples of resonance• A non vibrating tuning fork, having a natural

frequency of 256 Hz, will resonate when a vibrating tuning fork with a natural frequency of 256 Hz is brought near it.

• It is possible for an opera singer to Shattering a glass by maintaining a note with a frequency equal to the natural frequency of the glass.

• Collapse of the Tacoma Narrows Bridge due to high wind induced resonance.

• Pushing a child on the swing with the same rhythm as the swing will make the swing go higher.

• In conclusion, resonance occurs when two interconnected objects share the same vibrational frequency. When one of the objects is vibrating, it forces the second object into vibrational motion. The result is a large vibration. And if a sound wave within the audible range of human hearing is produced, a loud sound is heard.

In conclusion

example• In a demonstration, a vibrating tuning

fork causes a nearby second tuning fork to begin to vibrate with the same frequency. Which wave phenomenon is illustrated by this demonstration?

1. the Doppler effect 2. nodes 3. resonance 4. interference

example

• Which phenomenon occurs when an object absorbs wave energy that matches the object's natural frequency?

1. reflection

2. diffraction

3. resonance

4. interference

example• A student in a band notices that a drum

vibrates when another instrument emits a certain frequency note. This phenomenon illustrates

1. reflection

2. resonance

3. refraction

4. diffraction

Standing Wave Patterns• All objects have a frequency or set of frequencies with which they

naturally vibrate when struck, plucked, strummed or somehow disturbed. Each of the natural frequencies at which an object vibrates is associated with a standing wave pattern. When an object is forced into vibrations at one of its natural frequencies, it vibrates in a manner such that a standing wave is formed within the object.

• A standing wave pattern a vibrational pattern created within a medium when the reflected waves from one end of the medium interfere with incident waves from the source, The result of the interference is that specific points along the medium appear to be standing still while other points vibrated back and forth. Such patterns are only created within the medium at specific frequencies of vibration. These frequencies are known as harmonic frequencies or merely harmonics. At any frequency other than a harmonic frequency, the interference of reflected and incident waves results in a disturbance of the medium that is irregular and non-repeating.

• The natural frequencies of an object are merely the harmonic frequencies at which standing wave patterns are established within the object. These standing wave patterns represent the lowest energy vibrational modes of the object.

• The wave pattern associated with the natural frequencies of an object is characterized by points that appear to be standing still.

• The points in the pattern that are standing still are referred to as nodal points. These positions occur as the result of the destructive interference of incident and reflected waves. Each nodal point is surrounded by antinodal points.

Chapter 10 – Part II - Light

• Wave-like Behaviors of Light – Reflection, refraction, diffraction, Doppler

Effect

• Two Point Source Interference

• Thin Film Interference

• Light is a electromagnetic, transverse wave

• Polarization of light

Wavelike Behaviors of Light• An age-old debate which has persisted among

scientists is related to the question, "Is light a wave or a stream of particles?" Very noteworthy and distinguished physicists have taken up each side of the argument, providing a wealth of evidence for each side. The fact is that light exhibits behaviors which are characteristic of both waves and particles.

• Light exhibits certain behaviors which are characteristic of any wave.– Light reflects in the same manner that any wave

would reflect. – Light refracts in the same manner that any wave

would refract.– Light diffracts in the same manner that any wave

would diffract.– Light undergoes interference in the same manner

that any wave would interfere. – Light exhibits the Doppler effect just as any wave

would exhibit the Doppler effect.

Reflection of Light Waves• The reflection of light waves off of a mirrored surface

results in the formation of an image. One characteristic of wave reflection is that the angle at which the wave approaches a flat reflecting surface is equal to the angle at which the wave leaves the surface. This characteristic is observed for water waves and sound waves. It is also observed for light waves. Light, like any wave, follows the law of reflection when bouncing off surfaces.

Refraction of Light Waves• Refraction is bending of a wave’s path as it

passes from one medium to another medium. • This behavior of wave refraction is caused by

change of speed as the wave travels from one medium to another medium.

Blue Skies• The interaction of sunlight with matter can result in one of three wave

behaviors: absorption, transmission, and reflection. • the two most common types of matter present in the atmosphere are

gaseous nitrogen and oxygen. These particles are most effective in scattering the higher frequency and shorter wavelength portions of the visible light spectrum such as blue and violet light. This scattering process involves the absorption of a light wave by an atom followed by reemission of a light wave in a variety of directions.

• So as white light (ROYGBIV) from the sun passes through our atmosphere, the high frequencies (BIV) become scattered by atmospheric particles while the lower frequencies (ROY) are most likely to pass through the atmosphere without a significant alteration in their direction.

• All high frequency light are scattered. However, our eyes are more sensitive to light with blue frequencies. Thus, we view the skies as being blue in color.

What Makes a Red Sunset? • There are two reasons:

1. As the Sun gets lower in the sky, its light is passing through more of the atmosphere to reach you. Even more of the blue light is scattered, allowing the reds and yellows to pass straight through to your eyes.

2. Sometimes the whole western sky seems to glow. The sky appears red because larger particles of dust, pollution, and water vapor in the atmosphere reflect and scatter more of the reds and yellows.

• The refraction of light explains – visibility of the sun after it has actually

disappeared below the horizon.

The refraction of light explains mirages

• A mirage is a naturally occurring optical phenomenon in which light rays are bent to produce a displaced image of distant objects or the sky.

– mirages

• The "water" is actually a reflection of the blue sky, but a close look at this image also shows reflections of a car, power poles, bushes, a mile marker, and roadside grass. Because the reflection occurs solely at very shallow angles, the mirage appears only in the distance and continually recedes as one moves towards it.

Diffraction of Light Waves• Diffraction involves a change in direction of waves as

they pass through an opening or around an obstacle in their path. Both water waves and sound waves have the ability to travel around corners, around obstacles and through openings.

• Light does not exhibit a very noticeable ability to bend around the obstacle and fill in the region behind it with light. Nonetheless, light does diffract around obstacles.

Interference of light waves• Wave interference is a phenomenon which occurs when

two waves meet while traveling along the same medium. The interference of waves causes the medium to take on a shape which results from the net effect of the two individual waves upon the particles of the medium.

• Constructive interference occurs at any location along the medium where the two interfering waves have a displacement in the same direction.

• Destructive interference occurs at any location along the medium where the two interfering waves have a displacement in the opposite direction.

Two-Point Source Light Interference Patterns

• Light is a wave. Whenever light constructively interferes (such as when a crest meeting a crest or a trough meeting a trough), the two waves act to reinforce one another and to produce a "super light wave." On the other hand, whenever light destructively interferes (such as when a crest meets a trough), the two waves act to destroy each other and produce no light wave.

• If such an interference pattern could be created by two light sources and projected onto a screen, then there ought to be an alternating pattern of dark and bright bands on the screen.

Thin Film Interference• Perhaps you have witnessed streaks of color in a thin

film of oil resting upon a water puddle or concrete driveway. These streaks of color are the result of the interference of light by the very thin film of oil which is spread over the water surface. This form of interference is commonly called thin film interference and provides another line of evidence for the wave behavior of light.

Doppler Effect of Light

• Red shift: when stars are moving away.

• Blue shift: when starts are moving closer.

example

• Light from the star Betelgeuse displays a Doppler red shift.  This shift is best explained assuming that Betelgeuse is

1. decreasing in temperature

2. increasing in temperature

3. moving toward the Earth

4. moving away from the Earth

The Electromagnetic spectra• Electromagnetic waves are transverse waves which

are capable of traveling through a vacuum. Unlike mechanical waves which require a medium in order to transport their energy, electromagnetic waves are capable of transporting energy through the vacuum of outer space. Electromagnetic waves are produced by a vibrating electric charge and as such, they consist of both an electric and a magnetic component.

• The alternating electric field creates alternating magnetic field, and the alternating magnetic field creating alternating electric field.

• All electromagnetic waves, including light, travel at the same speed: 3 x 108 m/s in air or vacuum.

Visible Light is very small part of electromagnetic wave

• Unlike a mechanical wave, a light wave is an electromagnetic wave.

– Light waves are produced by vibrating electric charges. Light waves will have the same frequencies as the vibrating electron.

– Light waves are transverse waves which has both an electric and a magnetic component.

– Light waves can travel in vacuum.

Radio waves are electromagnetic waves

example

• Radio waves and gamma rays traveling in space have the same

1. frequency

2. wavelength

3. period

4. speed

example

• In a vacuum, all electromagnetic waves have the same

1. wavelength

2. frequency

3. speed

4. amplitude

example

• Compared to the speed of microwaves in a vacuum, the speed of x-rays in a vacuum is

1. less

2. greater

3. the same

example

• Radio wave A has a wavelength of 10. meters, and radio wave B has a wavelength of 12 meters. Compared to the speed of radio wave A in air, radio wave B’s speed in air is

1. lower

2. the same

3. higher

What is the wavelength of your favorite radio station?

• My favorite radio station is 100.7 (MHz) WHUD• By using v = fλ I can figure out their

wavelength

example

• To the nearest order of magnitude, how many times greater than the speed of sound is the speed of light?

example• A typical microwave oven produces

radiation at a frequency of 1.0x1010 hertz.  What is the wavelength of this microwave radiation?

Visible Light Spectrum• The visible light – the very narrow band of wavelengths in the

spectrum consists of wavelengths which range from approximately 700 nanometers (abbreviated nm) to approximately 400 nm. Expressed in more familiar units, the range of wavelengths extends from 7 x 10-7 meter to 4 x 10-7 meter. This narrow band of visible light is affectionately known as ROYGBIV.

• Each individual wavelength within the spectrum of visible light wavelengths is representative of a particular color.

..\..\RealPlayer Downloads\The Electromagnetic Spectrum-Visible light.flv

Dispersion• The separation of visible light into its different

colors is known as dispersion.

Different wavelength corresponds to different colors.

example1. A light wave is an electromagnetic wave which has both

an electric and magnetic component associated with it. Electromagnetic waves are often distinguished from mechanical waves. The distinction is based on the fact that electromagnetic waves ______.

a. can travel through materials and mechanical waves cannot

b. come in a range of frequencies and mechanical waves exist with only certain frequencies

c. can travel through a region void of matter and mechanical waves cannot

d. electromagnetic waves cannot transport energy and mechanical waves can transport energy

e. electromagnetic waves have an infinite speed and mechanical waves have a finite speed

example• Consider the electromagnetic spectrum as you

answer these three questions.a. Which region of the electromagnetic spectrum

has the highest frequency?

b. Which region of the electromagnetic spectrum has the longest wavelength?

c. Which region of the electromagnetic spectrum will travel with the fastest speed?

   

example

• A prism disperses white light, forming a spectrum. What is the best explanation for this phenomenon?

example

• Consider the visible light spectrum as you answer these two questions.

a. Which color of the visible light spectrum has the greatest frequency?

b. Which color of the visible light spectrum has the greatest wavelength?

example• Sunlight is composed of various intensities of all

frequencies of visible light. The graph represents the relationship between light intensity and frequency. Based on the graph, which color of visible light has the lowest intensity?

1. violet 2. red 3. blue 4. yellow 5. green 6. orange

• Sunlight is composed of various intensities of all frequencies of visible light. The graph represents the relationship between light intensity and frequency. It has been suggested that fire trucks be painted yellow green instead of red. Based on the information from the graph, which is the best explanation of the advantage of using yellow-green paint instead of red?

1. Yellow green is brighter than red. 2. Red has a higher frequency than yellow green. 3. Yellow green has a lower intensity than red. 4. The brightness of the colors is proportional to their

frequencies

example

• As the color of light changes from red to yellow, the frequency of the light

1. decreases

2. increases

3. remains the same

Laser

• LASER stands for - Light

Amplification by Stimulated Emission of

Radiation

• The emitted laser light is monochromatic and coherent, narrow low diverging beam

Monochromatic and coherent• Monochromatic means one color

• Coherent light are light waves that are "in phase" with one another.

• For example, two waves are coherent if the crests of one wave are aligned with the crests of the other and the troughs of one wave are aligned with the troughs of the other. Otherwise, these light waves are considered incoherent.

stimulated emission• When a photon hits an atom that is already excited, the atom

releases a new photon that is completely identical to the incoming photon; same color, going in the same direction. We call this process "stimulated emission".

example• What is one characteristic of a light beam

produced by a monochromatic laser?

1. It consists of coherent waves.

2. It can be dispersed into a complete continuous spectrum.

3. It cannot be reflected or refracted.

4. It does not exhibit any wave properties.

example

• Which term best describes the light generated by a laser?

1. diffused

2. coherent

3. dispersive

4. longitudinal

example

• A laser beam does not disperse as it passes through a prism because the laser beam is

1. monochromatic

2. polychromatic

3. polarized

4. longitudinal

example• Which diagram best represents light

emitted from a coherent light source? 

A B

DC

Light is a transverse wave that vibrate in multiple planes

• The transverse nature of an electromagnetic wave is quite different from any other type of wave which has been discussed so far. Light vibrates in multiple planes that is perpendicular to the wave’s motion.

A light wave which is vibrating in more than one plane is referred to as unpolarized light. Light emitted by the sun, by a lamp, or by a candle flame is unpolarized light. Such light waves are created by electric charges which vibrate in a variety of directions, thus creating an electromagnetic wave which vibrates in a variety of directions.

• It is possible to transform unpolarized light into polarized light. Polarized light waves are light waves in which the vibrations occur in a single plane.

• The most common method of polarization involves the use of a Polaroid filter. Polaroid filters are made of a special material which is capable of blocking one of the two planes of vibration of an electromagnetic wave. When unpolarized light is transmitted through a Polaroid filter, it emerges with one-half the intensity and with vibrations in a single plane; it emerges as polarized light.

Note: a longitudinal wave can not be polarized. Sound wave is a longitudinal wave, sound wave can not be polarized.

example

• The diagram shows a beam of light entering and leaving a "black box."  The box most likely contains a

1. prism

2. converging lens

3. double slit

4. polarizer

example

• Which phenomenon does not occur when a sound wave reaches the boundary between air and a steel block?

1. reflection

2. refraction

3. polarization

4. absorption

example

• The diagram shows sunglasses being used to eliminate glare. Which phenomenon is being represented in the diagram?

1. dispersion

2. diffraction

3. internal reflection

4. polarization

example

• Polychromatic light passing through a glass is separated into component frequencies. This phenomenon is called

1. diffraction

2. dispersion

3. reflection

4. polarization

Applications of Polarization• Used in glare-reducing sunglasses. • Light reflected from surfaces such as a flat road or

smooth water is generally horizontally polarized. This means that, instead of light being scattered in all directions in more usual ways, reflected light generally travels in a more horizontally oriented direction. This creates an annoying and sometimes dangerous intensity of light that we experience as glare.

• Polarized lenses contain a special filter that blocks this type of intense reflected light, reducing glare.

• Polarization is also used in the entertainment industry to produce and show 3-D movies. Three-dimensional movies are actually two movies being shown at the same time through two projectors.

..\..\RealPlayer Downloads\Polarized 3-D Glasses.flv

• Our model of the polarization of light provides some substantial support for the wavelike nature of light. It would be extremely difficult to explain polarization phenomenon using a particle view of light. Polarization would only occur with a transverse wave. For this reason, polarization is one more reason why scientists believe that light exhibits wavelike behavior.

• Reflection

• Refraction

• Interference

• Diffraction

• Doppler effect

• Polarization

In conclusion, wave is light because wave exhibit Wavelike Behaviors