3 generation and detection

7/30/2019 3 Generation and Detection

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Physics 1230: Light and Color

How do we generate light? How do we detect light? Concept of resonance

Instructor: Joseph MaclennanTOPIC 3 - Resonance and the

Generation of Light

http://www.colorado.edu/physics/phys1230


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In vacuum speed= c =distance

time=

T= f


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Static magnetic fields

Magnets generate magnetic fields that we can draw as field lines Magnetic field lines form continuous loops

Bar magnet The Earth


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Static electric fields

Positive and negative charges generate electric fields that we can drawas field lines Electric field lines from charges behave like stretched strings Electric fields generate static sparks: first radio transmissions

Positive chargehttp://library.thinkquest.org/10796/ch12/ch12.htm

Negative charge


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How do we generate a wave on a rope?

A) Drop one endB) Raise and lower one end quicklyC) Drop both endsD) Twang it in the center

http://phet.colorado.edu/new/simulations/sims.php?sim=Wave_on_a_String


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How do we generate EM waves? How do we make waves in the electromagnetic field? Lets wiggle the charges... Light waves are disturbances in the electromagnetic field, a

non-material physical entity whose equilibrium state is vacuum

http://phet.colorado.edu/new/simulations/sims.php?sim=Radio_Waves_and_Electromagnetic_Fields

Heinrich Hertz(1888)


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http://micro.magnet.fsu.edu/primer/java/polarizedlight/emwave/index.html

Electromagnetic Wave Propagation

Electromagnetic waves can be generated by a variety of methods, such as a

discharging spark or by an oscillating electrons in an atom or molecular. As thecurrent oscillates up and down in the spark gap, at a characteristic circuitfrequency, a magnetic field is created that oscillates in a horizontal plane. Thechanging magnetic field, in turn, induces an electric field so that a series ofelectrical and magnetic oscillations combine to produce a formation thatpropagates as an electromagnetic wave.


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Sources of electromagnetic waves

1. Heat

2. An oscillating electric field due to current changes(e.g. radio, TV, microwave oven)

3. Electrons in an excited atom(e.g. neon sign, or a fluorescent light)

4. Chemical excitation(e.g. firefly, phosphorescence)

So electrons moving up and down will emit electromagnetic wavesbecause the electrons make an electric field, and the lines arewiggled when the electrons move

What makes electrons move?


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Concept Quiz: How does your cell phone transmityour voice?

A) Generating sound wavesB)

Generating heat

C) Generating electromagnetic waves(high frequency radio)

D) Generating neural activity


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How does your cell phone generate electromagnetic waves?

A) HeatB) Voltage in a circuitC) Excited atomsD) Neural activity/heat


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Generating electromagnetic waves

SOURCE

Radio stations

Microwaves

Human body

Cell phones

Neon light

Laser

Warm stove

MECHANISM

Voltage in a circuit


Neural activity/heat


Excited atoms

Excited atoms

Heat


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Sources of electromagnetic waves

What are the frequencies of these sources?

Radio stations

Microwaves

Human body

Cell phones

Neon light

Laser

Warm stove


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http://micro.magnet.fsu.edu/primer/java/fluorescence/exciteemit/index.html

Creating Light from Atoms

Electrons can absorb energy from external sources, such as lasers, arc-discharge lamps, and tungsten-halogen bulbs, and be promoted to higher energylevels. Light energy is absorbed by an electron to elevate it into a higherenergy level and the energy can subsequently be released, in the form of alower energy photon, when the electron falls back to the original ground state.The precise difference between the energy levels determines the resonancefrequency or color of light that is emitted or absorbed.


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The emission and reception of electromagnetic waves is not

equally efficient for all waves, but depends on the natureof the emitter and receptor. One phenomenon, calledresonance, is responsible for this selectivity.

Resonance effects cause molecules to vibrate - forexample in your eye. Light of certain frequency(wavelength) drives the molecules to vibrate, so thateventually a signal is transmitted to your brain via the opticnerve.

Detecting Electromagnetic Waves Resonance


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Detection of electromagnetic waves -

Resonance and SelectivityWhy do we see only certain colors?

How does tuning of a radio signal work?


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In 1940, the Tacoma Narrows Bridge failed from wind-induced torsional oscillations. Research of design flaws inthe bridge led to the use of aerodynamic testing as astandard procedure in suspension span structural analysis.

Tacoma Narrows Bridge(State Route 16 spanning the Tacoma Narrows)

Can you think of other examples of resonance?


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Every material (such as glass, steel, concrete) has a natural frequency at which it vibrates,called a resonant frequency. If you put energy into the substance at its resonant frequency,

you will force it to vibrate or resonate (resonance is a forced vibration). In the case of thewine glass, your finger slides and sticks along the surface of the glass as you rub the rim (awet fingertip has no oil and makes a better contact with the glass). The rubbing impartsenergy to the glass molecules and causes them to resonate. The motion of your hand sets up awave of vibration traveling through the glass. The vibrating glass causes air molecules tovibrate at the same frequency. The vibrating air molecules are the sound wave that you hear(the frequency or pitch of the sound wave is the same as the resonant frequency of the

glass).

So, how does the water change the pitch of the singing wine glass? As the resonant wavemoves around the glass, it drags the water molecules with it, creating a wave of water thatyou can see near the edge of the glass. The dragging water molecules effectively increase themass (both the water and the glass molecules) and reduce the energy of the wave travelingthrough the glass. When the energy is reduced, so is the frequency of the wave in the glass,

which is reflected in the pitch of the sound wave that you hear.

If you impart enough energy to the glass at its resonant frequency, you can cause the glass toshatter. However, this takes more energy than you can provide by rubbing the rim. Somesingers can sing a note equal to the resonant frequency of a wine glass and cause it to shatter

Resonance

http://static.howstuffworks.com/mpeg/wine.mpg


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RESONANCE IN SOUND

http://www.colorado.edu/physics/2000/microwaves/standing_wave2.html

If you look at a guitar string under a strobe

light (or even a fluorescent light) you can seeit that makes a standing wave. Anotherexperiment you can do is to stand in theshower (they reflect sound well) and startsinging while changing the pitch slowly. At

certain pitches the sound will suddenlyamplify, because the sound waves fit an evennumber of times between the walls.


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Guitar string - can be tuned by changing the tension

Electrical circuits in radios

Nerve cells in eye - sensitive to red, green

and blue

Chemical dyes

Examples of Resonance


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If a singer can match the natural frequency of

the wine glass she can put more energy into thewine glass than it can handle. As the energybuilds, the glass begins to deform beyond whatits bonds can sustain. The trick is to sing withthe right frequency and being able to sustainthat note. It is not about singing loudly orhorribly. It is also not possible for one note toshatter all glasses as each glass would have itsown natural frequency.

http://www.blazelabs.com/pics/glass.movhttp://video.google.com/videoplay?docid=-7765557442856739526

Using Resonance to Shatter a Wineglass


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Using Resonance to shatter a Kidney stone.By tuning ultrasound waves to the natural frequency of akidney stone, we can rely on resonance to pulverize the stone

Another Example of Resonance


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You are swinging back and forth on a swing at thenatural frequency. If a friend (or your cat) joinsyou on the swing, the new natural frequency will be:

A. greaterB. the sameC. smallerD. zero - you wont be able to swing any more

Concept Test on Resonance


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If you rub the rim of a wineglass you can make itsing. This is because:

A. vibrations from your finger excite a resonantresponse in the glass

B. you need to sing the same note to get the resonanceC. the glass is a mechanical system in resonanceD. the table top transmits a musical tone to the glassE. otherwise it would shatter

Concept Test on Resonance


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http://micro.magnet.fsu.edu/primer/java/photosynthesis/index.html

RESONANT ABSORPTION OF LIGHT -Photosynthesis

Green plants absorb water and carbon dioxide

from the environment, and utilizing energy from

the sun, turn these simple substances into

glucose and oxygen. With glucose as a basic

building block, plants synthesize a number of

complex carbon-based biochemicals used togrow and sustain life. This process is termed

photosynthesis, and is the cornerstone of life on

Earth. In the applet, water molecules are converted to

molecular hydrogen and oxygen as a result of

photon absorption in the granum. Subsequently,

the hydrogen molecules react with carbon

dioxide in the stroma to produce oxygen and

carbohydrates.


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http://photoscience.la.asu.edu/photosyn/education/photointro.html

Color of Plants- Photosynthesis

Chlorophylls absorb blue and red lightand carotenoids absorb blue-greenlight, but green and yellow light arenot effectively absorbed byphotosynthetic pigments in plants;therefore, light of these colors iseither reflected by leaves or passes

through the leaves. This is why plantsare green.

Examples of photosyntheticorganisms: leaves from higherplants flanked by colonies ofphotosynthetic purple bacteria(left) and cyanobacteria (right).


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http://photoscience.la.asu.edu/photosyn/education/photointro.html

COLOR OF PLANTS - PhotosynthesisChlorophylls absorb blue and red

light and carotenoids absorb blue-

green light, but green and yellow

light are not effectively absorbed

by photosynthetic pigments in

plants; therefore, light of thesecolors is either reflected by leaves

or passes through the leaves. This is

why plants are green.

Absorption spectrum of isolated chlorophyll and

carotenoid species. The color associated with the

various wavelengths is indicated above the graph.


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What we see


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What is a resonance?

Many objects oscillateorvibrateat special frequenciescalled resonant frequenciesorresonances

When these objects are hitor "shaken" by an externalagent at a frequency = totheir resonant frequencythey will oscillate at their

resonant frequency. Hand moving back and forth atsame frequency as pendulumsresonant frequency (or hit)

Tacoma narrows bridge in thewind

Car on a dirt road with regularbumps (washboard effect)

The oscillations of the objectare largest when the"shaking" occurs at theobjects resonant frequency. We then say that a resonance

has occurrede.g. girl on swing being pushed byher mother (mothers pushfrequency = swing frequency)

Energy is transferred froman external agent to theobject during resonance. Wineglass broken by an opera

singers voice

due to resonance between voicesound frequency and natural

frequency of wineglass


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Effect of resonance produced by militaryhelicopter blade going around at frequency

resonant with the helicopter body


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What do resonances have to do

with light? When light is absorbed

by atoms we can thinkof this as a resonance The light frequency may

match a certain frequencyof resonant vibration in theatom.

When this happens, theenergy of the light istransferred to the atomand the light disappears.

For example, we see lightrays of 470 nm coming intoour eyes because this lightexcites a resonance incertain atoms inside oureyes

When light is emittedby atoms we can thinkof this as a resonance For example when an

electron hits an atom theatom can gain energy inthe form of resonances.

This energy in the atomcan then be released byanother resonant

interaction in which lightis emitted and the atomloses energy.

Each color of light emittedcorresponds to aparticular atomicresonance.


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Resonance

and theCreation of Light

Emission of light Absorption of light

3 generation and detection

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