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Sound and Light
Eleanor Roosevelt High SchoolChin-Sung Lin
Lesson 23
Sound Waves
Sound Waves• Sound as a mechanical wave:
All sounds are produced by the vibrations of material objects
• Sound as a longitudinal waveThe 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 waveCompression 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 = f λ = λ / T
Wavelength, How Long the Wave Is
Amplitude, AHow High the Wave Is
Single Frequency, fHow Many Wave Vibrations Each Second
Sound and Temperature
• Speed of sound vs. temperature1) 331 m/s in air at 0o C2) Changes by 0.607 m/s for every oC from 0oC
v = 331 m/s + (0.607 m/s°C) T
where v speed of sound [m/s]T temperature
[oC]• Subsonic – slower• Supersonic – faster than sound • Mach 1 = speed of sound
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 m-n 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 blue and green makes cyan (light blue)
Adding all three makes white
again
Adding red and green makes yellow
• 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
White light
Only red light is reflected
Colors
Colors
Purple light
Colors
Colors
White light
White light
Colors
White light
Blue light
Colors
Blue light
Shirt looks black
Shorts look blue
Colors
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
Umbra
Penumbra
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