chapter 30 light emission radio waves are produced by electrons moving up and down an antenna....
TRANSCRIPT
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Chapter 30
Light Emission
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Radio waves are produced byelectrons moving up and down an antenna.
Visible light is produced byelectrons changing energy states in an atom.
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1. EXCITATION
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Excitation - occurs when an electron
in an atom is given energy causing it to
jump to a higher orbit.
This can happen through
collisions or
photon absorption
(the photon absorption must exactly match the energy jump).
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e-
Here comes a nucleusWith possible orbits for electrons
e-
e-
Here comes a photon
Now the atom is said to be excited becausethe electron is in a higher than normal orbit.In this configuration the atom is not excited.Now the atom is in a non-excited state again.It emitted a photon when it changed orbits.
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The excited atom usually de-excites in
about 100 millionth of a second.
The subsequent emitted radiation has an
energy that matches that of the orbital
change in the atom.
Video - Atomic ExcitationVideo - Atomic Excitation
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This emitted radiation gives the
characteristic colors of the element
involved.
The atoms do not “wear out.”
Demo - Flame ColorsDemo - Flame Colors
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Classical explanation fails - it says
there should be radiation even when
there has been no change in energy
levels.
The electron should spiral into the
nucleus - the ultraviolet
catastrophe.
E = hf (h = Planck’s constant)
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Emission SpectraEmission Spectra
Continuous Emission Spectrum
Prism
Photographic Film
Slit
White Light Source
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Emission Spectra of Emission Spectra of HydrogenHydrogen
Prism
Photographic Film
Film
Slit
Low DensityGlowing
Hydrogen Gas
Discrete Emission Spectrum
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Slide - Spectra of Some GasesSlide - Spectra of Some Gases
Demo - Spectra of a Discharge Tube Gas Demo - Spectra of a Discharge Tube Gas
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2. INCANDESCENCE
Demo - Spectrum of Incandescent BulbDemo - Spectrum of Incandescent Bulb
Blue hot is hotter than white hot which is hotter than
red hot.
White light - all colors in the visible are present.
Electron transitions occur not only in the parent atom
but in adjacent atoms as well.
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Frequency
Rel
ativ
e E
ner
gy
(measured in Kelvins) Tf
Brightness versus color curve for different
temperatures
Peak Frequency
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Discrete Absorption Spectrum
Absorption Spectraof Hydrogen
Prism
Photographic Film
Film
Slit
White Light Source
Discrete Emission Spectrum
Hydrogen Gas
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Absorption Spectra
Frequencies of light that represent the correct
energy jumps in the atom will be absorbed.
When the atom de-excites, it emits the same kinds
of frequencies it absorbed.
However, this emission can be in any direction.
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Close inspection of the absorption spectrum
of the sun reveals missing lines known as
Fraunhofer lines.
In 1868 a pattern of lines was observed in
the solar spectrum that represented an
element that had not been found on earth. It
wasHelium - named for Helios, the sun.
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Doppler shifts are observed in the spectra of stars.
If a star is approaching, its spectra will be
blue shifted. If a star is moving away, its spectra will be
red shifted.Most spectra are red shifted indicating that on the average the universe is expanding.
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3. FLUORESCENCE
Some materials that are excited by UV
emit visible.
These materials are referred to as
fluorescent materials.
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Fluorescent Lamps
Primary excitation - electron collisions with low
pressure Hg vapor, UV given off
Secondary excitation - UV photons absorbed by
phosphors. Phosphors fluoresce emitting visible
light.
Remember that the visible light from the excited
mercury vapor is also emitted.
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4. PHOSPHORESCENCE Electrons get "stuck" in excited states in the atoms and de-
excitation occurs at different times for different atoms.
A continuous glow occurs for some time.
Demo - Fluorescent & Phosphorescent Demo - Fluorescent & Phosphorescent
Objects (including helicopter and Objects (including helicopter and
“ “Starry, Starry Night”)Starry, Starry Night”)
Bioluminescence
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Light
Amplification by
Stimulated
Emission of
Radiation
E5. LASERSLAS R
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Lasers produce coherent light.
Coherent light is light with the same
frequency and the same phase.
Demo - Making Laser Beam VisibleDemo - Making Laser Beam Visible
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Early model - ruby crystal doped with Cr
Slide - Helium-Neon LaserSlide - Helium-Neon Laser
Video - LasersVideo - Lasers
Today's models include gas (He-Ne), glass,
chemical, liquid, semiconductor, and x-ray.
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Applications:
Surveying Surgery Communication Computer storage Pollution detection 3-d TV (holograms) Thermonuclear fusion - heat H2 pellets
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Chapter 30 Review QuestionsChapter 30 Review Questions
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In fluorescence which has the In fluorescence which has the higher energy, higher energy, the radiation radiation absorbed or the radiation of an absorbed or the radiation of an emitted photon?emitted photon?
(a) absorbed
(b) emitted
(c) they both have the same energy
(a) absorbed
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What type of spectrum would you What type of spectrum would you expect to obtain if white light is expect to obtain if white light is shined through sodium vapor?shined through sodium vapor?
(a) an emission spectrum of sodium
(b) an absorption spectrum of sodium
(c) a continuous spectrum
(b) an absorption spectrum of sodium
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What causes laser light to have What causes laser light to have all of its waves moving in the all of its waves moving in the same direction?same direction?
(a) the mirrors in the laser
(b) the stimulated emission of the atoms to radiate in the same direction
(c) atoms are lined-up in the crystal so that they emit light only in one direction
(a) the mirrors in the laser
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Each element gives off a Each element gives off a characteristic color when it characteristic color when it undergoes the excitation and de-undergoes the excitation and de-excitation process. The color is excitation process. The color is due todue to(a) fluorescence
(b) secondary emission
(c) electrons moving to lower energy states
(d) refraction of light through the characteristic gas
(e) electrons giving off Cerenkov radiation
(c) electrons moving to lower energy states
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Which phenomenon has Which phenomenon has electrons getting "stuck" in electrons getting "stuck" in excited states?excited states?
(a) incandescence
(b) fluorescence
(c) phosphorescence
(c) phosphorescence