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Chapter 30
Light Emission
Radio waves are produced by
electrons moving up and down
an antenna.
Visible light is produced by
electrons 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).
Here comes a nucleus
With possible orbits for electrons
Here comes a photon
e-
ee-
In
Now
thisthe
the
configuration
atom isis in
said
to be
atom
excited
is not
because
excited.
Now
atom
a the
non-excited
state
again.
electron
is in a higher
normal
orbit.
Itthe
emitted
a photon
when itthan
changed
orbits.
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 Excitation
 This
emitted radiation gives the
characteristic colors of the element
involved.
 The
atoms do not “wear out.”
 Demo
- 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 Spectra
Continuous Emission Spectrum
Slit
White Light
Source
Prism
Photographic Film
Emission Spectra of
Hydrogen
Discrete Emission Spectrum
Slit
Film
Low Density
Glowing
Hydrogen Gas
Prism
Photographic Film
 Slide
- Spectra of Some Gases
 Demo
- Spectra of a Discharge Tube Gas
2. INCANDESCENCE
 Demo
 Blue
- Spectrum of Incandescent Bulb
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.
Brightness versus color curve for different
temperatures
Relative Energy

Peak Frequency
Frequency
f T
(measured in Kelvins)
Absorption Spectra
of Hydrogen
Discrete Emission Spectrum
Discrete Absorption Spectrum
Slit
Hydrogen Gas
Film
White Light
Source
Prism
Photographic Film
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.
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. PHOSPHORESCENCE
 Electrons
get "stuck" in excited states in the
atoms and de-excitation occurs at different
times for different atoms.
 A continuous
 Demo
glow occurs for some time.
- Fluorescent & Phosphorescent
Objects (including helicopter and
“Starry, Starry Night”)
 Bioluminescence
5. LA
LASERS
SER
Light
Amplification by
Stimulated
Emission of
Radiation
Lasers produce coherent light.
Coherent light is light with the same
frequency and the same phase.
Demo - Making Laser Beam Visible
 Early
model - ruby crystal doped with Cr
 Slide
- Helium-Neon Laser
 Video
- Lasers
 Today's
models include gas (He-Ne), glass,
chemical, liquid, semiconductor, and x-ray.
Applications:
Surveying
Surgery
Communication
Computer
storage
Pollution detection
3-d TV (holograms)
Thermonuclear fusion - heat H2
pellets
Chapter 30 Review Questions
In fluorescence which has the
higher energy, the radiation
absorbed or the radiation of an
emitted photon?
(a) absorbed
(b) emitted
(c) they both have the same energy
What type of spectrum would you
expect to obtain if white light is
shined through sodium vapor?
(a) an emission spectrum of sodium
(b) an absorption spectrum of sodium
(c) a continuous spectrum
What causes laser light to have all
of its 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
Each element gives off a
characteristic color when it
undergoes the excitation and deexcitation process. The color is
due 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
Which phenomenon has electrons
getting "stuck" in excited states?
(a) incandescence
(b) fluorescence
(c) phosphorescence