Physics Study Notes
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Lesson 20 Sound and Light
The Origin of Sound
a.
All sounds are produced by the vibrations of material objects.
b.
The original vibration stimulates the vibration of something larger or more massive, and then this vibrating
material sends a disturbance through a surrounding medium, usually air, in the form of longitudinal waves.
c.
The longitudinal waves flow through the air with a frequency equal to that of the vibrating source.
d.
A higher pitched sound has a higher vibration frequency.
e.
A young person can hear pitches with frequencies from about 20 to 20,000 Hz. Sound waves with
frequencies below 20 Hz are called infrasonic, and those with frequencies above 20,000 Hz are called
ultrasonic.
Sound in Air
a.
Compression: the pulse of compressed air is called compression.
b.
Rarefaction: the pulse of lower-pressure air is called rarefaction.
c.
For all the wave motions, it is not the medium travels across the space, but a pulse that travels.
d.
As the sound source vibrates, a series of compressions and rarefactions is produced.
Media that Transmit Sound
a.
Most sounds we hear are transmitted through the air. But sound also travels in solids and liquids. Solids and
liquids are generally good conductors of sound. In general, the speed of sound is highest in solids, and then
in liquids, and still slower in gases.
b.
Sound cannot travel in a vacuum. The transmission of sound requires a medium. If there is nothing to
compress and expand, there can be no sound. There may still be vibrations, but there is no sound.
Speed of Sound
a.
We hear thunder after we see a flash of lightening (unless we are at the source). Sound is much slower than
light.
b.
The speed of sound in dry air at 0oC is about 330 m/s, or about 1,200 km/hr, about one-millionth the speed
of light. Water vapor in the air increases this speed slightly. Increased temperature increases the speed of
sound also since the fast-moving molecules in warm air bump into each other more often and therefore can
transmit a pulse in less time. For each degree increase in air temperature above 0oC, the speed of sound in
air increases by 0.6 m/s. So in air at a normal room temperature of about 20oC, sound travels at about 340
m/s.
c.
The speed of sound in a material depends not on the material’s density, but on its elasticity. Elasticity is not
“stretchability”. Elasticity is ability of a material to change shape in response to an applied force, and then
resume its initial shape once the distorting force is removed.
d.
Steel is very elastic; putty is inelastic. In elastic materials, the atoms are relatively close together and
respond quickly to each other’s motion, transmitting energy with little loss.
e.
Sound travels about 15 times faster in steel than in air, and about 4 times faster in water than in air.
Loudness
a.
The intensity of a sound is proportional to the square of the amplitude of a sound wave. Loudness is a
physiological sensation sensed in the brain. It differs for different people. Loudness is subjective but is
related to sound intensity.
b.
Loudness varies nearly as the logarithm of intensity (power of ten). The unit of intensity for sound is the
decibel (dB).
c.
Starting with zero at the threshold of hearing for a normal ear, an increase of each 10 dB means that sound
intensity increases by a factor of 10. A sound of 10n dB is 10n times as intense as sound of 0 dB.
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Physics Study Notes
d.
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The sensation of loudness follows this decibel scale, i.e., human hearing is approximately logarithmic.
Forced Vibration
a.
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Lesson 20 Sound and Light
When a music instrument is mounted on a sounding board, and the sounding board has larger surface that
sets more air in motion. Thus the sound becomes very loud. This is a case of forced vibration.
Natural Frequency
a.
Objects vibrate differently when they strike the floor. When any object composed of an elastic material is
disturbed, it vibrates at its own special set of frequencies, which together form its special sound. An
object’s natural frequency depends on factors such as the elasticity and shape of the object.
b.
A natural frequency is one at which minimum energy is required to produce forced vibrations. It is also the
frequency that requires the least amount of energy to continue this vibration.
Resonance
a.
When the frequency of a forced vibration on an object matches the object’s natural frequency, a dramatic
increase in amplitude occurs. This phenomenon is called resonance.
b.
In order for something to resonate, it needs a force to pull it back to its starting position and enough energy
to keep it vibrating.
c.
A common experience illustrating resonance occurs on a swing. When pumping a swing, you pump in
rhythm with the nature frequency of the swing. More important than the force with which you pump is the
timing. Even small pumps if delivered in rhythm with the natural frequency of the swinging motion,
produce large amplitudes.
d.
A pair of tuning forks are adjusted to the same frequency and spaced apart. When one of the forks is struck,
it sets the other fork into vibration. This is a resonance. This is because the pushes occurs at the right time
and are repeatedly in the same direction as the instantaneous motion of the fork.
e.
When we tune our radio set, we are adjusting the natural frequency of the electronics in the set to match
one of many incoming signals. The set then resonates to one station at a time, instead playing all stations at
once.
f.
Resonance is not restricted to wave motion. It occurs whenever successive impulses are applied to a
vibrating object in rhythm with its natural frequency.
Interference
a.
Interference can occurs for both transverse and longitudinal waves. When the crest of one wave overlaps
with the crest of another, there is a constructive interference. When the crest of one wave overlaps with the
trough of another, there is a constructive interference. For sound, the crest of a wave corresponds to a
compression, and the trough of a wave corresponds to a rarefaction.
b.
Interference affects the loudness of sounds. If you are equally distant from two sound speakers that
simultaneously trigger identical sound waves of constant frequency, the sound is louder because the
compressions and rarefactions arrive in phase. If you move to the side so that paths from speakers differ by
a half-wavelength, rarefactions from one speaker reach you at the same time as compression from the
other. There is a cancellation.
c.
Destructive sound interference is a useful property in anti-noise technology. Sound compressions (or
rarefactions) from the noise source (such as jackhammer) are neutralized by mirror-image sound signal
compressions (or rarefactions). Noise-canceling earphones and electronic mufflers are the real-life
applications.
10 Beats
a.
When two tones of slightly different frequency are sounded together. A fluctuation in the loudness of the
combined sounds is heard. This periodic variation in the loudness of sound is called beats.
b.
If the frequency of the first sound is m, and the frequency of the second is n, a beat frequency of m-n is
heard.
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Physics Study Notes
Lesson 20 Sound and Light
c.
The beat waveform is produced by the interference of two superposed waveforms.
d.
If we overlap two combs of different teeth spacing, we will see a moiré pattern.
e.
Beats are a practical way to compare frequencies. When the frequencies are identical, the beats
disappeared.
11 The Sound of Music
a.
Music consists of a pleasing succession of pitches (frequencies). Music pitches are usually selected from a
specific sequence called a scale. In western music, the 12-note scale consists of a sequence of 12 pitches,
each of which is the twelfth root of 2 times the frequency of the next lower note. The 13th note has twice the
frequency of the first note and thus sounds an octave higher.
b.
To set up a continuous sound, it is necessary to set up a standing wave. Three large classes of traditional
musical instruments differ from one another in how they produce standing waves.
i)
Stringed instrument: such as guitar, violin and pianos producing standing waves in a tightly stretched
string.
ii) Percussion instrument: such as drums, gongs and bells producing standing wave through the vibration
of solid objects.
iii) Wind instrument: such as organs, trumpets and flutes, the standing wave is set up in the air enclosed in
the hollow tube.
c.
Some of the standing waves that can fit on a string of length L
are shown in the diagram. The wave with wavelength 2L is
called the fundamental, or first harmonic. Waves of
wavelength L , 2 L , 1 L , etc. can also fit on the string. Each of
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these higher harmonic or overtones corresponding to different
pitches. A complex wave is made up of a fundamental tone
€ several overtones. The distinctive timbres of different
and
€ €
musical instruments are a consequence of different relative
intensities of these overtones.
d.
Adding waves of different harmonic together can complex
sound wave. Based on this principle we can synthesize the
sounds of all kinds of different musical instruments.
e.
The technique of taking complex wave and breaking down into
a sum of simple, single frequency waves is called Fourier
analysis.
12 Introduction to Light
a.
The only thing we can really see is light.
b.
The source of light: sun, brightness of the sky, flames, whit-hot
filaments in lamps, glowing gases in the gases tubes, etc.
c.
Some materials (such as air, water and glass) allow light to pass through, other materials (such as thin paper
and frosted glass) allow the passage of light in diffused direction, and most materials do not allow the
passage of any light.
13 Early Concepts of Light
a.
Some ancient Greek philosopher thought that light is consists of tiny particles which could enter our eye to
create the sensation of vision.
b.
Others, including Socrates, Plato and Euclid, thought that vision resulted from streamer or filaments
emitted from the eye making contact with an object.
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Physics Study Notes
Lesson 20 Sound and Light
c.
Up until the time of Newton and beyond, most philosophers and scientists thought that light consists of
particles.
d.
However, one Greek, Empedocles, taught that light traveled in waves. A Dutch scientist Christian Huygens
also argued that light was a wave.
e.
Particle theory: Light seemed to move in straight lines instead of spread out as wave do.
f.
Wave theory: Huygens provided evidence of diffraction (light does spread out). The wave theory became
the accepted theory in the 19th century.
g.
In 1905 Einstein published a theory explaining the photo electric effect. According to this theory, light
consists of particles- massless bundles of concentrated electromagnetic energy (photons).
14 The speed of Light
a.
Olaus Roemer’s Observation—
i)
Io
Jupiter
1675 Roemer measure the period of Jupiter’s moons. The innermost moon, Io,
was measured to revolve around Jupiter in 42.5 hours. He found that while
Earth was moving away from Jupiter, the measured period of Io is longer than
average. When Earth was moving toward Jupiter, the measured period is
shorter than average.
ii) Christian Huygens correctly interpreted this discrepancy. When Earth is farther
away from Jupiter, it was the light that was late, not the moon. Because the
light has to travel the extra distance across the diameter of Earth’s orbit.
Earth
iii) Now we know that the extra distance is 300,000,000 km, so
Speed of light
Sun
= (extra distance traveled) / (extra time measured)
= 300,000,000 km/1,000 s
Earth
six month later
= 300,000 km/s
b.
Michelson-Morley’s experiment—
i)
1880 Michelson-Morley experiment measured
the speed of light. The speed of light is always
the same— 299,920 km/s. Simplified diagram
of the experiment:
Observer
Mirror
35 km away
Spinning
Octagonal
Mirror
ii) The speed of light in a vacuum is a universal
Light
constant. The light can travel around Earth 7.5
Source
times per second. Light take 8 minutes to travel
from sun to Earth, and 4 years from the next nearest star, Alpha Centauri. The distance light travels in
one year is called a light-year.
15 Electromagnetic Waves
a.
Light is energy that is emitted by accelerating electric charges in atoms. The energy travels in
electromagnetic wave.
b.
Light is a small portion of the electromagnetic spectrum which includes radio waves, microwaves,
infrared, light, ultraviolet, x-rays and gamma rays. All the waves have different frequencies and
wavelengths; all have the same speed.
c.
The electromagnetic waves of frequencies lower than the red of visible light are called infrared. The
electromagnetic waves of frequencies higher than the violet of visible light are called ultraviolet.
16 Light and the Transparent Materials
a.
When light is incident upon matters, electrons in the matter are forced into vibration. How a receiving
material responds when light is incident upon it depends on the frequency of light and the natural
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Physics Study Notes
Lesson 20 Sound and Light
frequency of electrons in the material. Visible light vibrates at a very high rate (1014 Hz). Electrons have a
small enough mass (very little inertia) to vibrate this fast.
b.
The natural vibration frequencies of an electron depend on how strongly it is attached to a nearby nucleus.
c.
Electrons in glass have a natural vibration frequency in the ultraviolet range. When ultraviolet light shines
on glass, resonance occurs as the wave build and maintain a large vibration between the electron and the
atomic nucleus. Since the frequency of light match the natural frequency of the electrons, the amplitude of
the vibration is unusually large. The atoms typically hold on to this energy for quite a long time. During
this time the atom collides with other atoms and give up its energy in the form of heat. That’s why glass is
not transparent to ultraviolet.
d.
When the visible light shins on glass, the electrons are forced into vibration with smaller amplitudes. The
atom holds the energy for less time, with less chance of collision with neighboring atoms, and less energy
is transferred as heat. The energy of the vibrating electrons is reemitted as transmitted light. Glass is
transparent to all the frequency of the visible light. The frequency of the reemitted light passed from atom
to atom is identical to the original one. The main difference is the slight time delay between absorption and
reemission.
e.
The time delay results in a lower average speed through a transparent material. In water light travels at
0.75c. In glass light travels at 0.67c. In diamond light travels at 0.4c. When light emerges from these
materials into the air, it travels at its original speed, c.
f.
Infrared waves vibrate not only the electrons, but also the entire structure of the glass. This vibration of the
structure increases the internal energy of the glass and makes it warmer.
g.
In sum glass is transparent to visible light, but not to ultraviolet and infrared light.
17 Opaque Materials
a.
Most materials absorb light without reemission and thus allow no light through them. They are opaque. In
opaque materials, any coordinated vibrations given by light are turned into random kinetic energy
(internal energy) and makes the materials slightly warmer.
b.
Metals are also opaque, but metals have lots of free electrons. When light shins on metal, and set these free
electrons into vibration, their energy does not sprint from atom to atom in the material, but is reemitted as
visible light. That’s why metals are shiny.
c.
The atmosphere is transparent to visible light and some infrared, but fortunately, almost opaque to highfrequency ultraviolet waves. The small amount that does get through is responsible for the sunburns.
Clouds are semitransparent to ultraviolet, which is why we can get a sunburn on a cloudy day. Ultra violet
also reflects from sand and water, which is why you can sometimes get a sunburn while in the shade of a
beach umbrella.
18 Shadows
a.
A thin beam of light is often called a ray.
b.
When light shines on an object, a shadow is formed where light rays cannot reach. Sharp shadows are
formed by a small light source nearby or a larger source far away. When the projection plane is closer to
the object, the shadow is also sharper.
c.
There is usually a dark part on the inside and a lighter part around the edges. A total shadow is called an
umbra, and a partial shadow a penumbra.
d.
Solar eclipse—When moon passes between Earth and the sun, because of the large size of the sun, the ray
taper to provide an umbra and a surrounding penumbra. If you stand in the umbra part of the shadow, you
experience a brief darkness of the day. If you stand in the penumbra, you experience a partial eclipse.
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Physics Study Notes
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Lesson 20 Sound and Light
Lunar eclipse—When moon passes into the shadow of Earth, we have lunar eclipse. Whereas a solar
eclipse can be observed only in a small region of Earth at a given time, a lunar eclipse can be seen by all
observers on the nighttime-half of Earth.
19 Polarization
a.
Light waves are transverse. A single vibrating electron emits an electromagnetic wave that is polarized.
Polarized light lies along the same plane as that of the vibrations of the electron that emits it. A vertically
vibrating electron emits light that is vertically polarized, while a horizontally vibrating electron emits light
that is horizontally polarized.
b.
A common light source is not polarized. A polarizing filter has a polarization axis that is in the direction of
the vibrations of the polarized light wave. When common light shines on a polarizing filter, the light that is
transmitted is polarized.
c.
Light will pass through a pair of polarizing filters
when their polarization axes are aligned, but not
when they are crossed at right angles.
d.
Light that reflects at glancing angles from
nonmetallic surfaces, such as glass, water or road,
vibrates mainly in the plane of the reflecting
surface. So glare from a horizontal surface is
horizontally polarized. That’s why the axes of
polarized sunglasses are vertical. In this way the glare from horizontal surfaces is eliminated.
20 Polarized Light and 3-D Viewing
a.
3-D vision depends on both eyes viewing a scene from slightly different angles. A pair of photographs or
movie frames, taken a short distance apart (about average eye spacing), can be seen in 3-D when the left
eye sees only the left view and the right eye sees only the right view.
b.
3-D movies are accomplished by projecting a pair of views through polarization filters onto a screen. The
polarization axes are at right angles to each other. The overlapping pictures look blurry to the naked eye.
But when viewers wear polarizing eyeglasses with the lens axes also at right angles, viewers will feel the
depth.
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