chapter 8 lecture conceptual integrated science second edition © 2013 pearson education, inc....

Chapter 8 Lecture

ConceptualIntegrated Science

Second Edition

© 2013 Pearson Education, Inc.

Waves—Sound and Light


This lecture will help you understand:

• Vibrations and Waves• Wave Motion• Transverse and Longitudinal Waves• The Nature of Sound• Resonance• The Nature of Light• Reflection• Transparent and Opaque Materials• Color• Refraction• Diffraction• Interference• The Doppler Effect• The Wave–Particle Duality


Vibrations and Waves

• A vibration is a wiggle in time.

• A wave is a wiggle in space and time—a disturbance that travels from one place to another transporting energy.



• A vibration is described in terms of frequency—how frequently vibratory motion occurs.

• A wave is described in terms of frequency, speed, amplitude, and wavelength.



• Frequency:– The number of to-and-fro vibrations in a given

time– Unit: 1 vibration per second = 1 Hertz

• Period:– The time it takes for a complete vibration– Unit: any unit of time, often the second



• Relationship between frequency and period:– Frequency = 1/period– Unit: Hertz (Hz)– Period = 1/frequency– Unit: second (s)

• The source of all waves is a vibration.• Higher frequency means increased rate of

energy transfer. Pulses occur more frequently and produce waves that are more closely spaced—shorter wavelengths.


Vibrations and WavesCHECK YOUR NEIGHBOR

If the frequency of a particular wave is 20 Hz, its

period is

A. 1/20 second.

B. 20 seconds.

C. more than 20 seconds.

D. none of the above

Explain your answer to your neighbor.


Vibrations and WavesCHECK YOUR ANSWER If the frequency of a particular wave is 20 Hz, itsperiod is

A. 1/20 second.B. 20 seconds. C. more than 20 seconds.D. none of the above

Explanation:

When f = 20 Hz, T = 1/f = 1/(20 Hz) = 1/20 second.


Wave Motion

• Wave motion is the propagation of a disturbance through a medium.

• The medium transporting the wave returns to its initial condition after the disturbance has passed.

• Wave motion requires an energy source and a medium (except for light) through which the energy is transferred.


Wave Motion

• Wave characteristics:– Crest—highest point– Trough—lowest point– Wavelength– Amplitude– Frequency– Period


Wave MotionCHECK YOUR NEIGHBOR

The distance between adjacent peaks in the

direction of travel for a transverse wave is its

A. frequency.

B. period.

C. wavelength.

D. amplitude.



Wave MotionCHECK YOUR ANSWER The distance between adjacent peaks in the direction oftravel for a transverse wave is its A. frequency.B. period. C. wavelength.D. amplitude.

Explanation:The wavelength of a transverse wave is also the distancebetween adjacent troughs, or between any adjacentidentical parts of the waveform


Wave Motion

• Wave speed– describes how fast the disturbance moves

through the medium. – is related to the frequency and wavelength of

the wave.

• Equation for wave speed:– Wave speed = frequency wavelength

v =


Transverse and Longitudinal Waves

• Two different types of waves are classified by the direction in which the medium vibrates compared to the direction of energy travel.– Transverse wave: Vibration is at right angles

(sideways) to wave travel.– Longitudinal wave: Vibration is in the direction of

travel.– Wave travel consists of compression and rarefaction

components.


Transverse and Longitudinal WavesCHECK YOUR NEIGHBOR

The vibrations along a transverse wave move in a

direction

A. along the wave.

B. perpendicular to the wave.

C. both of the above

D. neither of the above



Transverse and Longitudinal WavesCHECK YOUR ANSWERThe vibrations along a transverse wave move in a direction

A. along the wave.

B. perpendicular to the wave.

C. both of the above

D. neither of the above

Comment:

The vibrations in a longitudinal wave, in contrast, are along

(or parallel to) the direction of wave travel.


The Nature of Sound

• Sound travels in longitudinal waves consisting of vibrating compressions and rarefactions through the air.

• Speed of sound: Sound travels at 340 m/s in air at 20°C.


The Nature of SoundA situation to ponder…

• Consider a person attending a concert that is being broadcast over the radio. The person sits about 45 m from the stage and listens to the radio broadcast with a transistor radio over one ear and a nonbroadcast sound signal with the other ear. Further suppose that the radio signal must travel all the way around the world before reaching the ear.


A situation to ponder…CHECK YOUR NEIGHBOR

Which signal will the person hear first?

A. Radio signal

B. Nonbroadcast sound signal

C. Both at the same time




A situation to ponder…CHECK YOUR ANSWER Which signal will the person hear first?

A. Radio signalB. Nonbroadcast sound signalC. Both at the same timeD. none of the above

Explanation:A radio signal travels at the speed of light—3 108 m/s. Time to travel 45 m at 340 m/s ≈ 0.13 s. Time to travel 4 107 m (Earth’s circumference) at 3 108 m/s ≈ 0.13 s.So, if you sit farther back at the concert, the radio signal will reach youfirst!


The Nature of Sound

• For each increase of 1°C above 0°C, the speed of sound increases by 0.6 m/s.

• Order of increasing speeds of sound:– In air (≈ 340 m/s)– In warm air (>340 m/s)– In water (≈ four times speed in air)– In steel (≈ 15 times speed in air)


Resonance

• Resonance occurs whenever successive impulses are applied to a vibrating object in rhythm with its natural frequency.


The Nature of Light

• Light is electromagnetic waves created by vibrating electric charges with frequencies that fall within the range of sight.

• The frequency of vibrating electrons equals the frequency of the light.

• Light travels nearly a million times

faster than sound in air.• Light and all electromagnetic

waves are transverse waves.


The Nature of Light

• The electromagnetic spectrum


The Nature of LightCHECK YOUR NEIGHBOR The electromagnetic spectrum is a span of electromagnetic

waves, ranging from lowest to highest frequencies. The

smallest portion of the electromagnetic spectrum is that of

A. radio waves.

B. microwaves.

C. visible light.

D. gamma rays.



The Nature of LightCHECK YOUR ANSWER The electromagnetic spectrum is a span of electromagneticwaves, ranging from lowest to highest frequencies. Thesmallest portion of the electromagnetic spectrum is that of A. radio waves.B. microwaves. C. visible light.D. gamma rays.

Explanation:The answer can be inferred from a careful study of thespectrum and its regions in Figure 8.10.


The Nature of Light

Order of increasing frequency of visible light:• Red• Violet—nearly twice the frequency of red• Ultraviolet—cause sunburns• X-rays• Gamma rays


The Nature of Light

• Electromagnetic waves are composed of perpendicular electric and magnetic fields that vibrate perpendicular to the direction of wave travel. The electric and magnetic fields regenerate each other by electromagnetic induction.


The Nature of LightA situation to ponder…

• A photographer wishes to photograph a lightning bolt by setting his camera so that it is triggered by the sound of thunder.


A situation to ponder…CHECK YOUR NEIGHBOR

Is this a good idea or a poor idea?

A. Good idea for nearby lightning strikes

B. Good idea for all strikes

C. Poor idea for nearby lightning strikes

D. Poor idea for all strikes



A situation to ponder…CHECK YOUR ANSWER Is this a good idea or a poor idea?

A. Good idea for nearby lightning strikes

B. Good idea for all strikes

C. Poor idea for nearby lightning strikes

D. Poor idea for all strikes

Explanation:

Light travels about a million times faster than sound. By the

time the sound of thunder arrives, the lightning bolt is long

gone.


Reflection

• Reflection is the returning of a wave to the medium through which it came when it encounter a reflective surface

• Law of reflection:– Angle of incidence = angle of reflection


Reflection

• Diffuse reflection– When light is incident on a rough surface, it is

reflected in many directions.


ReflectionCHECK YOUR NEIGHBOR Compared with a dry road, seeing is more difficult when

driving at night on a wet road. Why?

A. A wet surface is smooth with less diffuse reflection, part of which would otherwise reach the driver’s eyes.

B. A wet road usually means a wet windshield.

C. A wet road usually means more vapor in the air.

D. There is no reason—that’s just the way it is.



ReflectionCHECK YOUR ANSWERCompared with a dry road, seeing is more difficult when

driving at night on a wet road. Why?

A. A wet surface is smooth with less diffuse reflection, part of which would otherwise reach the driver’s eyes.

B. A wet road usually means a wet windshield.

C. A wet road usually means more vapor in the air.

D. There is no reason—that’s just the way it is.


Transparent and Opaque Materials

• Transparent materials– Light passes through transparent materials in

straight lines, with atoms undergoing a series of absorptions and reemissions.

Examples: glass, water


Transparent and Opaque Materials

• Opaque materials– Colored glass is opaque to much of incident

white light.


Color

• The Color that we see depends on the frequency of light, ranging from lowest (red) to highest (violet). In between are the colors of the rainbow.

• Hues in seven colors: red, orange, yellow, green, blue, indigo, and violet

• Grouped together, they add to appear white.


Color

• Selective reflection– Most objects don’t emit light, but reflect light.– A material may absorb some of the light and

reflect the rest.• Selective transmission

– The color of a transparent object depends on the color of the light it transmits.


Color

• Mixing Colored lights (integrated)• Three types of cone receptors in our eyes perceive color.• Each is stimulated by only certain frequencies of light.

– Light of lower frequencies stimulates the cones sensitive to low frequencies (red).

– Light of middle frequencies stimulates the cones sensitive to mid-frequencies (green).

– Light of high frequencies stimulates the cones sensitive to high frequencies (blue).

– when all three cones are stimulated equally, we see white light.


Color

• Additive primary colors (red, blue, green):– Red + blue = magenta– Red + green = yellow– Blue + green = cyan


Color

• Opposites of primary colors:– The opposite of green is magenta.– The opposite of red is cyan.– The opposite of blue is yellow.

• The addition of any color to its opposite color results in white.


ColorCHECK YOUR NEIGHBOR

To which radiation is the human eye blind?

A. Infrared

B. Ultraviolet

C. Both infrared and ultraviolet

D. Neither infrared nor ultraviolet



ColorCHECK YOUR ANSWER

To which radiation is the human eye blind?

A. Infrared

B. Ultraviolet

C. Both infrared and ultraviolet

D. Neither infrared nor ultraviolet


Refraction

• Refraction is the bending

of a wave due to a change

in the medium and/or

speed of the wave.


Refraction

• Examples of refraction:– When light slows down in

going from one medium to

another, as when going from

air to water, it bends toward

the normal.

– When light speeds up in traveling

from one medium to another, as

when going from water to air, it

bends away from the normal.


Refraction

• Sound waves refract when parts of the wave fronts travel at different speeds.

• Refraction occurs when sound waves are affected by uneven winds, or when air near the ground is warmer than the air above.


Refraction

• Lenses are classified into two kinds:– Converging lens: Incoming parallel light rays

refract and converge to a focal point.– Diverging lens: Incoming parallel light rays

refract in such a way that extended rays diverge to a focal point in front of the lens.


RefractionCheck Your Neighbor

Which of these occur in raindrops that form

rainbows?

A. Reflection

B. Refraction

C. Dispersion

D. all of the above



RefractionCheck Your Answer

Which of these occur in raindrops that form

rainbows?

A. Reflection

B. Refraction

C. Dispersion

D. all of the above


Diffraction

• Diffraction is any bending of light by means other than reflection and refraction.

• Smaller openings produce greater diffraction (greater bending of the waves at edges).

• The amount of diffraction depends on thewavelength of the wave compared to the size of the obstruction that casts the shadow.


Interference

• Interference is the combined

effect of two or more

overlapping waves.


Interference

• There are two types of interference:– In constructive interference,

the crest of one wave

overlaps the crest of another

wave. The individual effects

add, resulting in a wave of

increased amplitude.– In destructive interference, the crest of one

wave overlaps the trough of another. Individual effects are reduced.


InterferenceCHECK YOUR NEIGHBOR

Interference is a property of

A. sound.

B. light.

C. both sound and light.

D. neither sound nor light.



InterferenceCHECK YOUR ANSWER Interference is a property of A. sound.B. light. C. both sound and light.D. neither sound nor light.

Explanation:See Figure 8.47 for illustrations of both light and soundinterference. Interestingly, the presence of interference tellsa physicist whether something is wavelike or not. All typesof waves can interfere.


The Doppler Effect

• The Doppler effect is a change in frequency as measured by an observer due to the motion of the source or listener.

• The Doppler effect is named after Austrian physicist and mathematician Christian Johann Doppler.


The Doppler Effect

• Example of the Doppler effect:– The frequency of waves received by an

observer increases as a sound source moves toward the observer. The wave frequency decreases as the source moves away.


The Doppler EffectCHECK YOUR NEIGHBOR

When a fire engine approaches you, the

A. speed of its sound increases.

B. frequency of its sound increases.

C. wavelength of its sound increases.

D. all of the above.



The Doppler EffectCHECK YOUR ANSWER When a fire engine approaches you, the A. speed of its sound increases.B. frequency of its sound increases. C. wavelength of its sound increases.D. all of the above.

Comment:Be sure you distinguish between sound, speed,and sound frequency.


The Doppler EffectCHECK YOUR NEIGHBOR

The Doppler effect occurs for

A. sound.

B. light.

C. both sound and light.

D. neither sound nor light.



The Doppler EffectCHECK YOUR ANSWER The Doppler effect occurs for

A. sound.B. light. C. both sound and light.D. neither sound nor light.

Explanation:As the text states, the Doppler effect occurs for sound(Figure 8.58) and for light (see the IntegratedScience—Astronomy feature). Astronomers measure thespin rates of stars by the Doppler effect.


The Wave–Particle Duality

• In ancient times, Plato, other Greek philosophers, and Isaac Newton thought that light was composed of tiny particles.

• 100 years after Newton, Thomas Young demonstrated the wave nature of light with interference experiments.

• 25 years later, the wave view was confirmed by Heinrich Hertz.

• Later in 1905, Albert Einstein challenged the wave theory and stated that light was confined in tiny particles of energy called photons. His particle model of light was verified by the photoelectric effect.



• Today, light is acknowledged to have both a wave nature and a particlenature—wave–particle duality:– Light reveals itself as a wave or particle

depending on how it is being observed.– Light behaves as a wave when it is traveling

from a source to a place where it is detected, and light behaves as a stream of photons when it interacts with a detector.



• The photoelectric effect– When light shines on certain

metal surfaces, electrons areejected from those surfaces.

– Ultraviolet and violet light impart sufficient energy to knock electrons from those metal surfaces, but lower-frequencylight does not, even when it isvery bright.


The Wave–Particle DualityCHECK YOUR NEIGHBORWhy is unexposed black-and-white photographicfilm not "exposed" when in red light but is exposedwhen in white light? A. The red light in a dark room is usually too dim.B. Red light has insufficient energy per photon to

"expose" the film. C. Red light is low-temperature light.D. none of the above



The Wave–Particle DualityCHECK YOUR ANSWER

Why is unexposed black-and-white photographic

film not "exposed" when in red light but is exposed

when in white light?

A. The red light in a dark room is usually too dim.

B. Red light has insufficient energy per photon to "expose" the film.

C. Red light is low-temperature light.




Findings:

1. The ejection of electrons depends on only the frequency of the light.

2. The higher the frequency of the light, the greater the kinetic energy of the ejected electrons.

Explanation:

Electrons in the metal are bombarded by "particles of

light"—photons. The energy of each photon is proportional

to its frequency: E .


The Wave–Particle DualityCHECK YOUR NEIGHBOR

Light travels as a wave and is absorbed as

A. a wave.

B. a particle.

C. both a wave and a particle.

D. neither a wave nor a particle.



The Wave–Particle DualityCHECK YOUR ANSWER Light travels as a wave and is absorbed as

A. a wave.B. a particle. C. both a wave and a particle.D. neither a wave nor a particle.

Explanation:Light is wavelike as it travels but particle-like whenit encounters a surface.

chapter 8 lecture conceptual integrated science second edition © 2013 pearson education, inc....

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