Lesson 23
Sound and Light
Eleanor Roosevelt High School
Chin-Sung Lin
Sound Waves
Sound Waves
• Sound as a mechanical wave:
All sounds are produced by the vibrations of material objects
• Sound as a longitudinal wave
The motion of the individual particles of the medium is in a
direction which is parallel to the direction of energy transport
• Sound as a pressure wave
Compression and rarefaction
Compression and Rarefaction
• Compression
The pulse of compressed air is called compression
• Rarefaction
The pulse of lower-pressure air is called rarefaction
Speed of Sound
Frequency of Sound
• The frequency of sound equals to that of the
vibrating source
• Dynamic range of human ear: 20 ~ 20 kHz
• Infrasonic: f < 20 Hz
• Ultrasonic: f > 20 kHz
Speed of Sound
• Speed of sound = distance / time
v=d/t
distance d
Speed of Sound
• Speed of sound = frequency  wavelength
v=fλ=λ/T
Wavelength, 
How Long the Wave Is
Single Frequency, f
How Many Wave Vibrations Each Second
Amplitude, A
How High the Wave Is
Sound and Temperature
• Speed of sound vs. temperature
1) 331 m/s in air at 0o C
2) Changes by 0.607 m/s for every oC from 0oC
v = 331 m/s + (0.607 m/s °C)
where
v
T
speed of sound [m/s]
temperature [oC]
• Subsonic – slower
• Supersonic – faster than sound
• Mach 1 = speed of sound
T
Sound and Medium
• All sounds are produced by the vibrations of material
objects
• The transmission of sound requires a medium
• Sound cannot travel in a vacuum. There may still be
vibrations, but there is no sound
Sound and Elasticity
• The speed of sound in a material depends on its
elasticity (not density)
• 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
• Sound travels about 15 times faster in steel than in
air, and about 4 times faster in water than in air
SONAR
• Sonar (SOund Navigation And Ranging) is a technique
that uses sound propagation (usually underwater) to
navigate, communicate with or detect other vessels
Loudness
Loudness
• The amount of power per square meter is called the
intensity of the sound
• The intensity of a sound is proportional to the square
of the amplitude of a sound wave
• Loudness is a subjective sensation of people but is
related to sound intensity
• Human hearing is approximately logarithmic (power
of ten)
Loudness and Decibel (dB)
• The unit of intensity for sound is the decibel (dB)
• The scale begins (0 dB) on the softest sound (the
threshold of hearing) that a person can hear
• The scale ends (120 dB) the volume that causes pain (the
threshold of pain)
• 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
• The threshold of pain is 1,000,000,000,000 as great of
the threshold of hearing
Loudness and Decibel (dB)
• The decibel (dB) is a logarithmic unit that indicates the
ratio of a physical quantity (usually power or intensity)
relative to a reference level
• P1 and P0 must measure the same type of quantity, and
have the same units before calculating the ratio
• If P1 = P0 then LdB = 0
• If P1 > P0 then LdB > 0
• If P1 < P0 then LdB < 0
Loudness and Decibel (dB)
Resonance
Forced Vibrations
• 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
Natural Frequencies
• When any object composed of an elastic material is
disturbed, it vibrates at its own special set of
frequencies (together form its special sound)
• Depends on the elasticity and shape of the object
• A frequency at which minimum energy is required to
produce forced vibrations
• A frequency that requires the least amount of energy
to continue this vibration
Resonance
• When the frequency of a forced vibration on an
object matches the object’s natural frequency, a
dramatic increase in amplitude occurs
• A common experience illustrating resonance occurs
on a swing
Resonance
• A pair of tuning forks with the same frequency are
spaced apart
• When one of the forks is struck, it sets the other fork
into vibration. This is a resonance
Resonance
• 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
Resonance
• Wine glass can be shattered by human voice through
resonance
Resonance
• Resonance is not restricted to sound wave motion. It
occurs whenever successive impulses are applied to
a vibrating object in rhythm with its natural
frequency
Interference
Interference
• 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
Interference
• Interference affects the loudness of sounds
Anti-Noise Technology
• Destructive sound interference is a useful property in
anti-noise technology: Noise-canceling earphones
Anti-Noise Technology
• Destructive sound interference is a useful property in
anti-noise technology: electronic mufflers
Beats
• 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
• If the frequency of the first sound is m, and the
frequency of the second is n, a beat frequency of mn is heard
fbeat = | fm – fn |
Beats
fm
fn
fbeat = | fm – fn |
Beat Frequency
• Two tuning forks are sounded together producing 3 beats
per second. If the first fork has a frequency of 300 Hz, what
are the possible frequencies of the second fork?
Beat Frequency
• A tuning fork with a frequency of 256 Hz is sounded the
same time as a second tuning fork producing 20 beats in 4
seconds. What are the possible frequencies of the second
tuning fork?
Beats
• The beat waveform is produced by the interference of
two superposed waveforms
• Beats are a practical way to compare frequencies. When
the frequencies are identical, the beats disappeared
Sound of Music
The Sound of Music - Frequency
• Music consists of a pleasing succession of pitches
(frequencies). Music pitches are usually selected from a
specific sequence called a scale
The Sound of Music - Frequency
• The 12-note scale consists of a sequence of 12 pitches,
the 13th note has twice the frequency of the first note
• The frequencies 220Hz and 440Hz both correspond to
the musical note A, but one octave apart
The Sound of Music - Frequency
• Each of which is the
twelfth root of 2
times the frequency
of the next lower
note
The Sound of Music – Frequency
• The frequency of note A is 440 Hz, calculate the frequency
of note B
The Sound of Music - Standing waves
• 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
– Stringed instrument: in a tightly stretched string
– Percussion instrument: through the vibration of solid
objects
– Wind instrument: set up in the air enclosed in the
hollow tube
The Sound of Music - Standing waves
• Stringed instrument: in a tightly stretched string
The Sound of Music – Standing Waves
• λ = 2L
• λ=L
• λ=
(2/3)L
The Sound of Music – Standing Waves
• The wave with wavelength 2L is called the fundamental,
or first harmonic
• Each of these higher harmonic or overtones
corresponding to higher pitches (frequencies)
The Sound of Music - Standing waves
• Percussion instrument: through the vibration of solid
objects
The Sound of Music - Standing waves
• Wind instrument: set up in the air enclosed in the hollow tube
The Sound of Music – Speed of Sound
• The standing wave of the air column can be used to calculate
the speed of sound
The Sound of Music – Speed of Sound
• The first resonant length of an open pipe is 33.0 cm. If the
frequency of a sound resonating over this pipe is 512 Hz,
what is the speed of sound?
The Sound of Music – Speed of Sound
• A sound with a frequency of 560 Hz is traveling at 350 m/s.
What is the length of an open air column that resonates this
sound at its shortest resonant length?
The Sound of Music – Speed of Sound
• The air temperature in a room is 25oC. A tuning fork
resonates over a closed tube 30.0 cm long, its shortest
resonant length. What is the wavelength of the sound?
The Sound of Music – Timbre
• A complex wave is made up
of a fundamental tone and
several overtones
• The distinctive timbres of
different musical
instruments are a
consequence of different
relative intensities of these
overtones
Fourier Analysis
• The technique of taking
complex wave and breaking
down into a sum of simple,
single frequency waves is
called Fourier Analysis
Fourier Analysis
• A square wave can be view as the sum of a series of
sine waves of different frequencies
Fourier Analysis
• The mathematical tool to
convert signals from time
domain to frequency
domain is called Fourier
Transform
• Adding different
harmonic waves together
can make complex sound
wave. Based on this
principle we can
synthesize the sounds of
different musical
instruments
Light
The ONLY thing we can SEE is …...
Light
What is Light?
• Particle theory: Light seemed to move in straight
lines instead of spread out as wave do
What is Light?
• Wave theory: Dutch physicist Christiaan Huygens
provided evidence of diffraction (light does spread
out). The wave theory became the accepted theory
in the 19th century
What is Light?
• Photon Model: 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)
Photoelectric Effect
Photoelectric Effect
• The photoelectric effect refers to the emission of
electrons from the surface of a metal in response to
incident light
• Energy is absorbed by electrons within the metal, giving
the electrons sufficient energy to be 'knocked' out of the
surface of the metal
Photoelectric Effect
• Maxwell wave theory of light predicts that the more
intense the incident light the greater the average energy
carried by an ejected (photoelectric) electron
• Experiment shows that the energies of the emitted
electrons to be independent of the intensity of the
incident radiation
• Einstein (1905) resolved this paradox by proposing that
the incident light consisted of individual quanta, called
photons, that interacted with the electrons in the metal
like discrete particles, rather than as continuous waves
Photoelectric Effect
• For a given frequency of the incident radiation, each
photon carried the energy E = hf, where h is Planck's
constant and f is the frequency
Speed of Light
Speed of Light
• 1675 Roemer measure the period of Jupiter’s moon, Io,
was measured to revolve around Jupiter in 42.5 hours.
• While Earth was moving away from Jupiter, the period is
longer than average. When Earth was moving toward
Jupiter, the period is shorter than average
Speed of Light
• Christian Huygens: 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
• Now we know that the extra distance is 300,000,000 km,
Speed of light = (extra distance traveled) / (extra time
measured)
= 300,000,000 km/1,000 s
= 300,000 km/s
= 3 x 10 8 m/s
Speed of Light
• Michelson’s Interferometer: An interference pattern is
produced by splitting a beam of light into two paths,
bouncing the beams back and recombining them
Speed of Light
• Michelson’s Interferometer: Two light paths
Speed of Light
• Light waves require a medium, the "luminiferous ether".
Because light can travel through a vacuum, it was
assumed that the vacuum must contain the medium of
light.
Speed of Light
• 1887 Michelson-Morley’s experiment measured the
speed of light to understand the properties of ether
Speed of Light
• The most famous failed
experiment disapproved
the existence of ether. The
speed of light is always
the same— 299,920 km/s
• the speed of light in
vacuum was independent
of the speed of the
observer!
• Michelson won the Nobel
Prize in Physics in 1907
Electromagnetic Waves
Electromagnetic Waves
• Light is energy that is emitted by accelerating electric
charges in atoms. The energy travels in electromagnetic
wave
Electromagnetic Waves
• Light is a small portion of the electromagnetic spectrum
All the waves have different frequencies and wavelengths;
all have the same speed
Electromagnetic Waves
• Typical human eyes respond to wavelengths from about
390 to 750 nm, or In terms of frequency, 400–790 THz
• The frequencies lower than the red light are called infrared
• The frequencies higher than the violet light are called
ultraviolet
Light and Materials
Light and Transparent Materials
• When light is incident upon matters, electrons are forced
into vibration
• Visible light vibrates at a very high rate. Electrons have a
small enough mass (very little inertia) to vibrate this fast
• Material responds depending on the frequency of light and
the natural frequency of electrons in the material
• The natural frequencies of an electron depend on how
strongly it is attached to a nearby nucleus
UV Light and Transparent Materials
• Electrons in glass have a natural vibration frequency in the
short wavelength ultraviolet (UV) range
• When ultraviolet light shines on glass, resonance occurs.
The amplitude of the vibration is unusually large
• The atom collides with other atoms and give up its energy
in the form of heat
• Glass is not transparent to short wavelength ultraviolet
Visible Light and Transparent Materials
• 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,
and less energy is transferred as heat
• Glass is transparent to all the frequency of the visible light.
The energy of the vibrating electrons is reemitted as
transmitted 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
Speed of Light in Transparent Materials
• 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
Infrared Light and Transparent Materials
• 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
• In sum glass is transparent to visible light, but not to
ultraviolet and infrared light
Light and Opaque Materials
• 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 internal energy and makes the
materials slightly warmer
Light and Opaque Materials
• 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
Light and Opaque Materials
• The atmosphere is transparent to
visible light, but almost opaque to
high-frequency 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
• Ultraviolet also reflects from sand
and water, which is why you can get a
sunburn while under a beach
umbrella
Colors
Colors
• White light can be split up to make separate colours.
These colours can be added together again
• The primary colours of light are red, blue and green
Adding blue and red
makes magenta
(purple)
Adding red and
green makes
yellow
Adding blue and green
makes cyan (light blue)
Adding all three
makes white
again
Colors
• The colour an object appears depends on the colours of
light it reflects
White light
Colors
• The colour an object appears depends on the colours of
light it reflects
Only red light is
reflected
White light
Colors
Colors
Purple light
Colors
White light
Colors
White light
White light
Colors
Blue light
Colors
Shirt looks black
Blue light
Shorts look blue
Shadows
Shadows
• When light shines on an object, a shadow is formed where
light rays cannot reach
Shadows
• Sharp shadows are formed by a small light source nearby, a
larger source far away, or when the projection plane is
closer to the object
Shadows
• 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
Penumbra
Umbra
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
Solar Eclipse
• 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
Lunar Eclipse
• When moon passes into the shadow of Earth, we have
lunar eclipse
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
Polarization
Polarization
• Light waves are transverse. A single vibrating electron emits
an electromagnetic wave that is polarized along the same
plane as that of the vibrations of the electron that emits it
Polarization
• A horizontally vibrating electron
emits light that is horizontally
polarized
• A vertically vibrating electron
emits light that is vertically
polarized
Polarization
• 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
Polarization & 3D Movie
• 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
Polarization & 3D Movie
Polarization & 3D Movie
• 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. When viewers wear
polarizing eyeglasses with the lens axes also at right angles,
viewers will feel the depth
The End