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