Physics 1230: Light and Color

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Physics 1230: Light and Color

•  How do we generate light?

•  How do we detect light?

•  Concept of resonance

Instructor: Joseph Maclennan TOPIC 3 - Resonance and the

Generation of Light

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

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In vacuum

€

speed = c = distance

time = λ

T = λ ^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 draw as field lines

•  Electric field lines from charges behave like stretched strings

•  Electric fields generate static sparks: first radio transmissions

Positive charge

http://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 end

B)  Raise and lower one end quickly C)  Drop both ends

D)  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?

•  Let’s 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 the current oscillates up and down in the spark gap, at a characteristic circuit

frequency, a magnetic field is created that oscillates in a horizontal plane. The

changing magnetic field, in turn, induces an electric field so that a series of

electrical and magnetic oscillations combine to produce a formation that

propagates 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 waves because the electrons make an electric field, and the lines are wiggled when the electrons move

•  What makes electrons move?

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Concept Quiz: How does your cell phone transmit your voice?

A)  Generating sound waves B)  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)  Heat

B)  Voltage in a circuit C)  Excited atoms

D)  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 Voltage in a circuit Neural activity/heat Voltage in a circuit 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 energy levels. Light energy is absorbed by an electron to elevate it into a higher

energy level and the energy can subsequently be released, in the form of a

lower energy photon, when the electron falls back to the original ground state.

The precise difference between the energy levels determines the resonance

frequency 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 nature of the emitter and receptor. One phenomenon, called resonance, is responsible for this selectivity.

Resonance effects cause molecules to vibrate - for example in your eye. Light of certain frequency (wavelength) drives the molecules to vibrate, so that eventually a signal is transmitted to your brain via the optic nerve.

Detecting Electromagnetic Waves – Resonance

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Detection of electromagnetic waves - Resonance and Selectivity

Why 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 in the bridge led to the use of aerodynamic testing as a

standard 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 the wine glass, your finger slides and sticks along the surface of the glass as you rub the rim (a wet fingertip has no oil and makes a better contact with the glass). The rubbing imparts

energy to the glass molecules and causes them to resonate. The motion of your hand sets up a wave of vibration traveling through the glass. The vibrating glass causes air molecules to

vibrate 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 wave moves around the glass, it drags the water molecules with it, creating a wave of water that you can see near the edge of the glass. The dragging water molecules effectively increase the mass (both the water and the glass molecules) and reduce the energy of the wave traveling through 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 to shatter. However, this takes more energy than you can provide by rubbing the rim. Some

singers 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 see it that makes a standing wave. Another

experiment you can do is to stand in the shower (they reflect sound well) and start singing while changing the pitch slowly. At certain pitches the sound will suddenly

amplify, because the sound waves fit an even

number 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 the wine glass than it can handle. As the energy builds, the glass begins to deform beyond what its bonds can sustain. The trick is to sing with the right frequency and being able to sustain that note. It is not about singing loudly or horribly. It is also not possible for one note to shatter all glasses as each glass would have its own natural frequency.

http://www.blazelabs.com/pics/glass.mov

http://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 a

kidney 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 the natural frequency. If a friend (or your cat) joins you on the swing, the new natural frequency will be:

A.  greater B.  the same C.  smaller

D.  zero - you won’t 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 it

”sing”. This is because:

A.  vibrations from your finger excite a resonant response in the glass

B.  you need to sing the same note to get the resonance C.  the glass is a mechanical system in resonance

D.  the table top transmits a musical tone to the glass E.  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 to grow 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 light and carotenoids absorb blue-green light, but green and yellow light are not effectively absorbed by photosynthetic pigments in plants;

therefore, light of these colors is either reflected by leaves or passes through the leaves. This is why plants are green.

Examples of photosynthetic organisms: leaves from higher plants flanked by colonies of photosynthetic purple bacteria (left) and cyanobacteria (right).

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

COLOR OF PLANTS - Photosynthesis

Chlorophylls 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 these colors 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 oscillate or

vibrate at special frequencies called resonant frequencies or resonances

•  When these objects are hit or "shaken" by an external agent at a frequency = to their resonant frequency they will oscillate at their resonant frequency.

–  Hand moving back and forth at same frequency as pendulum’s resonant frequency (or hit) –  Tacoma narrows bridge in the

wind

–  Car on a dirt road with regular bumps (washboard effect)

•  The oscillations of the object are largest when the

"shaking" occurs at the

object’s resonant frequency.

–  We then say that a resonance has occurred

e.g. girl on swing being pushed by her mother (mother’s push

frequency = swing frequency)

•  Energy is transferred from an external agent to the object during resonance.

–  Wineglass broken by an opera singer’s voice

–  due to resonance between voice sound frequency and natural frequency of wineglass

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Effect of resonance produced by military helicopter blade going around at frequency

resonant with the helicopter body

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Physics 1230: Light and Color