Chapter 30
Light Emission
Radio waves are produced byelectrons moving up and down an antenna.
Visible light is produced byelectrons changing energy states in an atom.
1. EXCITATION
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).
e-
Here comes a nucleusWith possible orbits for electrons
In this configuration the atom is not excited.
Here comes a photon
In this configuration the atom is not excited.
e-
e-
Now the atom is said to be excited becausethe electron is in a higher than normal orbit.
Now the atom is in a non-excited state again.It emitted a photon when it changed orbits.
e-
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.
Atomic ExcitationAtomic Excitation
This emitted radiation gives the characteristic
colors of the element involved.
The atoms do not “wear out.”
Demo - Flame ColorsDemo - Flame Colors
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)
Emission SpectraEmission Spectra
Continuous Emission Spectrum
Prism
Photographic Film
Slit
White Light Source
Emission Spectra of Emission Spectra of HydrogenHydrogen
Prism
Photographic Film
Film
Slit
Low DensityGlowing
Hydrogen Gas
Discrete Emission Spectrum
Spectra of Some Gases – Next Slide Spectra of Some Gases – Next Slide
Demo - Spectra of a Discharge Tube Gas Demo - Spectra of a Discharge Tube Gas
and Sodium Vapor Lamp and Sodium Vapor Lamp
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.
Frequency
Rel
ativ
e E
ner
gy
(measured in Kelvins) f T
Brightness versus Color curve for different
temperatures
Peak Frequency
Discrete Absorption Spectrum
Absorption Spectraof Hydrogen
Prism
Photographic Film
Film
Slit
White Light Source
Discrete Emission Spectrum
Hydrogen Gas
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.
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 was
Helium - named for Helios, the sun.
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.
3. FLUORESCENCE
Some materials that are excited by UV emit
visible.
These materials are referred to as fluorescent
materials.
Demo – Fluorescence with Green Laser and Demo – Fluorescence with Green Laser and
Liquids Liquids
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.
4. PHOSPHORESCENCEElectrons 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 Objects
(including helicopter and (including helicopter and
“ “Starry, Starry Night”)Starry, Starry Night”)
Bioluminescence
Light
Amplification by
Stimulated
Emission of
Radiation
E5. LASERSLAS R
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
Early model - ruby crystal doped with Cr
Slide - Helium-Neon Laser - Helium-Neon Laser
LasersLasers
Chapter 30 Review QuestionsChapter 30 Review Questions
What type of spectrum would you expect What type of spectrum would you expect to obtain if white light is shined through to obtain if white light is shined through sodium vapor?sodium vapor?
(a) an emission spectrum of sodium
(b) an absorption spectrum of sodium
(c) a continuous spectrum
In fluorescence which has the higher In fluorescence which has the higher energy, the radiation absorbed or the energy, the radiation absorbed or the radiation of an emitted photon?radiation of an emitted photon?
(a) absorbed
(b) emitted
(c) they both have the same energy
Which phenomenon has electrons getting Which phenomenon has electrons getting "stuck" in excited states?"stuck" in excited states?
(a) incandescence
(b) fluorescence
(c) phosphorescence
What causes laser light to have all of its What causes laser light to have all of its waves moving in the same direction?waves moving in the 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
Link to Chapter 27
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