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Discharge LampsDischarge Lamps

Chapter 14 part2Chapter 14 part2

1020C1020C

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Introductory QuestionIntroductory Question

A fluorescent lamp tube is coated with a A fluorescent lamp tube is coated with a white powder on its inside surface. If that white powder on its inside surface. If that powder were not there, the lamp would powder were not there, the lamp would appearappear

A.A. brighterbrighter

B.B. dimmerdimmer

C.C. about the same overall brightness, but about the same overall brightness, but with an unpleasantly bright white line with an unpleasantly bright white line near its centernear its center

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Observations aboutObservations aboutDischarge LampsDischarge Lamps

They often take a few moments to turn onThey often take a few moments to turn on They come in a variety of colors, including They come in a variety of colors, including

whitewhite They are often whiter than incandescent They are often whiter than incandescent

bulbsbulbs They last longer than incandescent bulbsThey last longer than incandescent bulbs They sometimes hum loudlyThey sometimes hum loudly They flicker before they fail completelyThey flicker before they fail completely

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4 Questions about4 Questions aboutDischarge LampsDischarge Lamps

Why not stick with incandescent Why not stick with incandescent lamps?lamps?

How can colored lights mix so we How can colored lights mix so we see white?see white?

How can white light be produced How can white light be produced without heat?without heat?

How do gas discharge lamps How do gas discharge lamps produce their light?produce their light?

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Question 1Question 1

Why not stick with incandescent Why not stick with incandescent lamps?lamps?

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Shortcomings of Thermal Shortcomings of Thermal LightLight

Incandescent lamps are reddish and Incandescent lamps are reddish and inefficientinefficient

Filament temperature is too low, thus too redFilament temperature is too low, thus too red The temperature of sunlight is 5800 °CThe temperature of sunlight is 5800 °C The temperature of an incandescent lamp is 2500 The temperature of an incandescent lamp is 2500

°C°C An incandescent lampAn incandescent lamp

emits mostly invisible infrared light,emits mostly invisible infrared light, so less than 10% of its thermal power is visible so less than 10% of its thermal power is visible

light.light.

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Question 2Question 2

How can colored lights mix so we How can colored lights mix so we see white?see white?

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Seeing in ColorSeeing in Color

We have three groups of light-We have three groups of light-sensing cone cellssensing cone cells Primary sensors/colors of light: red, Primary sensors/colors of light: red,

green, and bluegreen, and blue When the primaries mix unevenly, When the primaries mix unevenly,

we see others colorswe see others colors When three primaries mix evenly, When three primaries mix evenly,

we see whitewe see white

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Question 3Question 3

How can white light be produced How can white light be produced without heat?without heat?

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Fluorescent Lamps Fluorescent Lamps (Part 1)(Part 1)

Fluorescent tubesFluorescent tubes contain low density gas and metal contain low density gas and metal

electrodes, electrodes, which inject free electric charges into which inject free electric charges into

the gasthe gas to form a plasma—a gas of charged to form a plasma—a gas of charged

particlesparticles so that electric fields cause current to so that electric fields cause current to

flow in plasma.flow in plasma.

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Fluorescent Lamps Fluorescent Lamps (Part 2)(Part 2)

Collisions in the plasma causeCollisions in the plasma cause electronic excitation in the gas atomselectronic excitation in the gas atoms and some ionization of the gas atoms,and some ionization of the gas atoms, which help to sustain the plasma.which help to sustain the plasma.

Excited atoms emit light through Excited atoms emit light through fluorescencefluorescence

Fluorescence is part of quantum Fluorescence is part of quantum physicsphysics

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Quantum Physics of Quantum Physics of AtomsAtoms

In an atom,In an atom, the negative electrons “orbit” the positive nucleusthe negative electrons “orbit” the positive nucleus and form standing waves known as orbitals.and form standing waves known as orbitals. Each orbital can have at most two electrons in itEach orbital can have at most two electrons in it

An electron in a specific orbital has a total energy,An electron in a specific orbital has a total energy, that is the sum of its kinetic and potential energies.that is the sum of its kinetic and potential energies.

An atom’s electrons An atom’s electrons are normally in lowest energy orbitals – the ground stateare normally in lowest energy orbitals – the ground state but can shift to higher energy orbitals – excited states.but can shift to higher energy orbitals – excited states.

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Atoms and LightAtoms and Light

Electron orbitals are standing waves,Electron orbitals are standing waves, which do not change measurably with timewhich do not change measurably with time and therefore do not involve charge motionand therefore do not involve charge motion and do not emit (or absorb) light.and do not emit (or absorb) light.

While an electron is changing orbitals,While an electron is changing orbitals, there is charge motion and acceleration,there is charge motion and acceleration, so the electron can emit (or absorb) light.so the electron can emit (or absorb) light.

Such orbital changes are call radiative Such orbital changes are call radiative transitionstransitions

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Light from AtomsLight from Atoms

The wave/particle duality applies to light,The wave/particle duality applies to light, so light travels as a wave (diffuse rippling so light travels as a wave (diffuse rippling

fields)fields) but is emitted or absorbed as a particle (a but is emitted or absorbed as a particle (a

photon).photon). An atom’s orbitals differ by specific An atom’s orbitals differ by specific

energiesenergies and these energy differences set the photon and these energy differences set the photon

energies,energies, so an atom has a specific spectrum of photons.so an atom has a specific spectrum of photons.

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Photons, Energy, and Photons, Energy, and ColorColor

Photon’s frequency is proportional to its Photon’s frequency is proportional to its energyenergy

Photon energy = Planck constant· frequencyPhoton energy = Planck constant· frequency and and frequency· wavelength = speed of lightfrequency· wavelength = speed of light.. Each photon emitted by an atom hasEach photon emitted by an atom has

a specific energy,a specific energy, a specific frequency,a specific frequency, a specific wavelength (in vacuum),a specific wavelength (in vacuum), and a specific color when we detect it with our and a specific color when we detect it with our

eyes.eyes.

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Atomic FluorescenceAtomic Fluorescence Excited atoms lose energy via radiative transitionsExcited atoms lose energy via radiative transitions During a transition, electrons shift to lower During a transition, electrons shift to lower

orbitalsorbitals Photon energy is the difference in orbital energiesPhoton energy is the difference in orbital energies

Small energy differences Small energy differences infrared photons infrared photons Moderate energy differences Moderate energy differences red photons red photons Big energy differences Big energy differences blue photons blue photons Very Big differences Very Big differences ultraviolet photons ultraviolet photons

Each atom typically has a bright “resonance line”Each atom typically has a bright “resonance line” Mercury’s resonance line is at 254 nm, in the UVMercury’s resonance line is at 254 nm, in the UV

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PhosphorsPhosphors

A mercury discharge emits mostly UV lightA mercury discharge emits mostly UV light A phosphor can convert UV light to visibleA phosphor can convert UV light to visible

by absorbing a UV photonby absorbing a UV photon and emitting a less-energetic visible photon.and emitting a less-energetic visible photon. The missing energy usually becomes thermal The missing energy usually becomes thermal

energy.energy. Fluorescent lamps use white-emitting Fluorescent lamps use white-emitting

phosphorsphosphors Specialty lamps use colored light-emittersSpecialty lamps use colored light-emitters

Blue, green, yellow, orange, red, violet, etc.Blue, green, yellow, orange, red, violet, etc.

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Introductory Question Introductory Question (revisited)(revisited)

A fluorescent lamp tube is coated with a A fluorescent lamp tube is coated with a white powder on its inside surface. If that white powder on its inside surface. If that powder were not there, the lamp would powder were not there, the lamp would appearappear

A.A. brighterbrighter

B.B. dimmerdimmer

C.C. about the same overall brightness, but about the same overall brightness, but with an unpleasantly bright white line with an unpleasantly bright white line near its centernear its center

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Fluorescent Lamps Fluorescent Lamps (Part 3)(Part 3)

Starting a discharge requires electrons in the Starting a discharge requires electrons in the gasgas

Heated filaments can provide those electronsHeated filaments can provide those electrons Manual preheat lamps (initial filament heating)Manual preheat lamps (initial filament heating) Automatic preheat lamps (initial filament heating)Automatic preheat lamps (initial filament heating) Rapid start lamps (constant filament heating)Rapid start lamps (constant filament heating)

Rapid start lamps can be dimmed, unlike the othersRapid start lamps can be dimmed, unlike the others

High voltages can provide electronsHigh voltages can provide electrons Instant start lamps (high voltage pulse start)Instant start lamps (high voltage pulse start)

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Fluorescent Lamps Fluorescent Lamps (Part 4)(Part 4)

Gas discharges are unstableGas discharges are unstable Gas is initially insulatingGas is initially insulating Once discharge is started, gas become a Once discharge is started, gas become a

conductorconductor The more current it carries, the better it The more current it carries, the better it

conductsconducts Current tends to skyrocket out of controlCurrent tends to skyrocket out of control

Stabilizing discharge requires ballastStabilizing discharge requires ballast Inductor ballast (old, 60 Hz, tend to hum)Inductor ballast (old, 60 Hz, tend to hum) Electronic ballast (new, high-frequency, silent)Electronic ballast (new, high-frequency, silent)

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Question 4Question 4

How do gas discharge lamps How do gas discharge lamps produce their light?produce their light?

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Low-Pressure Discharge Low-Pressure Discharge LampsLamps

Mercury gas has its resonance line in the UVMercury gas has its resonance line in the UV Low-pressure mercury lamps emit mostly UV Low-pressure mercury lamps emit mostly UV

lightlight Some gases have resonance lines in the Some gases have resonance lines in the

visiblevisible Low-pressure sodium vapor discharge lampsLow-pressure sodium vapor discharge lamps

emit sodium’s yellow-orange resonance light,emit sodium’s yellow-orange resonance light, so they are highly energy efficientso they are highly energy efficient but extremely monochromatic and hard on the but extremely monochromatic and hard on the

eyes.eyes.

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Pressure BroadeningPressure Broadening

High pressures broaden each High pressures broaden each spectral linespectral line Collisions occur during photon Collisions occur during photon

emissions, emissions, so frequency and wavelength become so frequency and wavelength become

smeared out.smeared out. Collision energy shifts the photon Collision energy shifts the photon

energyenergy

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Radiation TrappingRadiation Trapping

Radiation trapping occurs at high Radiation trapping occurs at high atom densitiesatom densities Atoms emit resonance radiation very Atoms emit resonance radiation very

efficientlyefficiently Atoms also absorb resonance radiation Atoms also absorb resonance radiation

efficientlyefficiently Resonance radiation photons are trapped Resonance radiation photons are trapped

in the gasin the gas Energy must escape discharge via other Energy must escape discharge via other

transitionstransitions

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High-Pressure Discharge High-Pressure Discharge LampsLamps

At higher pressures, new spectral lines At higher pressures, new spectral lines appearappear

High-pressure sodium vapor discharge lampsHigh-pressure sodium vapor discharge lamps emit a richer spectrum of yellow-orange colors,emit a richer spectrum of yellow-orange colors, are still quite energy efficient,are still quite energy efficient, but are less monochromatic and easier on the but are less monochromatic and easier on the

eyes.eyes. High-pressure mercury discharge lampsHigh-pressure mercury discharge lamps

emit a rich, bluish-white spectrum,emit a rich, bluish-white spectrum, with good energy efficiency.with good energy efficiency. Adding metal-halides adds red to improve Adding metal-halides adds red to improve

whiteness.whiteness.

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Summary aboutSummary aboutDischarge LampsDischarge Lamps

Thermal light sources are energy Thermal light sources are energy inefficientinefficient

Discharge lamps produce more light, Discharge lamps produce more light, less heatless heat

They carefully assemble their visible They carefully assemble their visible spectraspectra

They use atomic fluorescence to create They use atomic fluorescence to create lightlight

Some include phosphors to alter colorsSome include phosphors to alter colors