Conceptual Physics
11th Edition
Chapter 30:
LIGHT EMISSION
© 2010 Pearson Education, Inc.
This lecture will help you understand:
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Excitation
Emission Spectra
Incandescence
Absorption Spectra
Fluorescence
Phosphorescence
Lamps
Lasers
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Light Emission
• Light emission is understood in
terms of the familiar planetary
model of the atom.
• Just as each element is
characterized by the number of
electrons that occupy the shells
surrounding its atomic nucleus,
each element also possesses
its own characteristic pattern of
electron shells, or energy states.
• These states are found only at
certain energies; we say they
are discrete. We call these
discrete states quantum states.
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Excitation
• When energy is imparted to an element,
an electron may be boosted to a higher
energy level. The atom is said to be
excited.
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Excitation
• Excitation
• The frequency of an emitted photon ~ energylevel difference in de-exciting:
E = hf
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Excitation
CHECK YOUR NEIGHBOR
Which has less energy per photon?
A.
B.
C.
D.
Red light
Green light
Blue light
All have the same.
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Excitation
CHECK YOUR ANSWER
Which has less energy per photon?
A.
B.
C.
D.
Red light
Green light
Blue light
All have the same.
Explanation:
In accord with E ~ f, the lowest-frequency light has the
lowest energy per photon.
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Excitation
CHECK YOUR NEIGHBOR
Excitation is the process in which
A.
B.
C.
D.
electrons are boosted to higher energy levels in
an atom.
atoms are charged with light energy.
atoms are made to shake, rattle, and roll.
None of the above.
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Excitation
CHECK YOUR ANSWER
Excitation is the process in which
A.
B.
C.
D.
electrons are boosted to higher energy levels in
an atom.
atoms are charged with light energy.
atoms are made to shake, rattle, and roll.
None of the above.
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Emission Spectra
Spectroscope
• Arrangement of slit, focusing lenses, and prism or
diffraction grating
• To see emission spectrum of light from glowing element
• When an electron is at a higher energy level, atom is
excited and temporarily loses the acquired energy when
it returns to a lower level and emits radiant energy.
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Emission Spectra
Spectral lines
• Forms an image of the slit on the screen using a
spectroscope
• Each component of color is focused at a definite
position according to frequency.
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Emission Spectra
Spectral lines of hydrogen
• More orderly than other elements
• Successive lines get closer until the lines merge.
• Swedish physicist and mathematician Johannes
Rydberg discovered that the sum of the frequencies of
two lines often equals the frequency of a third line.
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Emission Spectra
Ritz combination principle
• Rydberg’s discovery called the Ritz combination
principle:
The spectral lines of
any element include
frequencies that are
either the sum or the
difference of the
frequencies of two
other lines.
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Emission Spectra
CHECK YOUR NEIGHBOR
Most of what we know about atoms is gained by
investigating the
A.
B.
C.
D.
masses of elements.
electric charge of elements.
periodic table of the elements.
light they emit.
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Emission Spectra
CHECK YOUR ANSWER
Most of what we know about atoms is gained by
investigating the
A.
B.
C.
D.
masses of elements.
electric charge of elements.
periodic table of the elements.
light they emit.
Explanation:
Light emitted by atoms, their atomic spectra, are
considered to be the fingerprints of atoms.
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Incandescence
Incandescence
• The frequency of radiation emitted by a hot body
is proportional to the temperature of the hot
body.
f ~T
• Radiation curve of
brightness versus frequency

for emitted light.
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Incandescence
Incandescence (continued)
• Isolated bells ring with a
distinct frequency (as atoms
in a gas do).
• Sound from a box of bells
crowded together is
discordant (like light from an
incandescent solid).
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Incandescence
CHECK YOUR NEIGHBOR
Which lamp is more efficient for emitting light?
A.
B.
C.
D.
Incandescent lamp
Fluorescent lamp
Both the same for the same wattage
None of the above.
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Incandescence
CHECK YOUR ANSWER
Which lamp is more efficient for emitting light?
A.
B.
C.
D.
Incandescent lamp
Fluorescent lamp
Both the same for the same wattage
None of the above.
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Absorption Spectra
Absorption spectra
• Atoms in a gas absorb light of the same frequency they
emit.
• A spectroscope can detect “dark” lines in an otherwise
continuous spectrum.
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Fluorescence
Fluorescence
• Many materials excited by ultraviolet light emit
visible light upon de-excitation.
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Fluorescence
• Many substances undergo excitation when
illuminated with ultraviolet light.
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Fluorescence
• This excitation and de-excitation process is like leaping
up a small staircase in a single bound, and
• then descending one or two steps at a time rather than
leaping all the way down in a single bound.
• Photons of lower frequencies are emitted.
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Fluorescence
Fluorescent lamps
• UV emitted by excited gas strikes phosphor
material that emits white light.
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Fluorescence
CHECK YOUR NEIGHBOR
An atom that absorbs a photon can then emit one
A.
B.
C.
D.
only at the same energy.
of any energy depending on the situation.
only at a higher energy.
only at the same or lower energy.
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Fluorescence
CHECK YOUR ANSWER
An atom that absorbs a photon can then emit one
A.
B.
C.
D.
only at the same energy.
of any energy depending on the situation.
only at a higher energy.
only at the same or lower energy.
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Phosphorescence
• When excited, certain crystals as well as some
large organic molecules remain in a state of
excitation for a prolonged period of time.
• Unlike what occurs in fluorescent materials,
their electrons are boosted into higher orbits
and become “stuck.”
• As a result, there is a time delay between the
processes of excitation and de-excitation.
• Materials that exhibit this peculiar property are
said to have phosphorescence.
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Phosphorescence
• Atoms or molecules in these materials are excited by
incident visible light.
• Rather than de-exciting immediately, as fluorescent
materials do, many of the atoms remain in a
metastable state — a prolonged state of excitation—
sometimes as long as several hours, although most
de-excite rather quickly.
• If the source of excitation is removed—for instance, if
the lights are put out— an afterglow occurs while
millions of atoms spontaneously undergo gradual deexcitation.
• Many living creatures—from bacteria to fireflies and
larger animals, such as jellyfish—chemically excite
molecules in their bodies that emit light. Such living
things are bioluminescent.
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Lamps
Incandescent lamp
• A glass enclosure with a filament of
tungsten, through which an electric
current is passed.
• The hot filament emits a continuous
spectrum, mostly in the infrared, with
smaller visible part.
• The glass enclosure prevents oxygen in
air from reaching the filament, to prevent
burning up by oxidation.
• Argon gas with a small amount of a
halogen is added to slow the evaporation
of tungsten.
• The efficiency of an incandescent bulb is
10%
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Lamps
Fluorescent lamps
• UV emitted by excited gas strikes phosphor
material that emits white light.
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Lamps
Compact fluorescent lamps (CFL)
• Miniaturize a fluorescent tube,
wrap it into a coil, and outfit it
with the same kind of plug a
common incandescent lamp
has, and you have a compact
fluorescent lamp (CFL).
• CFLs are more efficient than
incandescent lamps, putting
out about 4 times more light
for the same power input.
• A downside to the CFL is its
mercury content, which poses
environmental disposal
problems.
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Lamps
Light-emitting diodes (LEDs)
• A diode is a two-terminal
electronic device that
permits a flow of charge in
only one direction.
• One kind of diode design is
the reverse of a photocell,
in that an impressed
voltage stimulates the
emission of light! This is a
light-emitting diode, LED.
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Lamps
LEDs (continued)
• A semiconductor layer
containing free electrons
is deposited onto the
surface of another
semiconductor that
contains energy “holes”
that can accept the free
electrons.
• An electrical barrier at
the boundary of these
two materials blocks
electron flow.
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Lamps
LEDs (continued)
• When an external voltage
is applied, the barrier is
overcome and energetic
electrons cross over and
“drop” into energy
“holes.”
• In a way similar to deexcitation, dropped
electrons lose potential
energy that is converted
into quanta of light—
photons.
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Lasers
Lasers
• Incoherent light (many frequencies and out of
phase)
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Lasers
Lasers (continued)
• Monochromatic light out of phase
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Lasers
Lasers (continued)
• Coherent light of identical frequencies in phase
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Lasers
Lasers (continued)
• A device that produces a beam of coherent light
• Many types and many
ranges of light
• Not a source of
energy (as is
sometimes thought)
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Lasers
The laser consists of a narrow Pyrex tube that
contains a low-pressure gas mixture consisting of
85% helium (small black dots) and 15% neon (large
colored dots).
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Lasers
Lasers (continued)
• When a high-voltage current zaps through the tube, it excites
both helium and neon atoms to their usual higher states, and
they immediately undergo de-excitation, except for one state
in the helium that is characterized by a prolonged delay
before de-excitation—a metastable state.
• Since this state is relatively stable, a sizable population of
excited helium atoms (black open circles) is built up.
• These atoms wander about in the tube and act as an energy
source for neon, which has an otherwise hard-to-come-by
metastable state very close to the energy of the excited
helium.
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Lasers
Lasers (continued)
• When excited helium atoms collide with neon atoms in their
lowest energy state (ground state), the helium gives up its
energy to the neon, which is boosted to its metastable state
(red open circles).
• The process continues, and the population of excited neon
atoms soon outnumbers neon atoms in a lower-energy
excited state.
• This inverted population is, in effect, waiting to radiate its
energy.
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Lasers
Lasers (continued)
• Some neon atoms eventually de-excite and radiate red
photons in the tube.
• When this radiant energy passes other excited neon atoms,
the latter are stimulated into emitting photons exactly in
phase with the radiant energy that stimulated the emission.
• Photons pass out of the tube in irregular directions, giving it
a red glow.
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Lasers
Lasers (continued)
• Photons moving parallel to the axis of the tube are reflected
from specially coated parallel mirrors at the ends of the
tube.
• The reflected photons stimulate the emission of photons
from other neon atoms, thereby producing an avalanche of
photons having the same frequency, phase, and direction.
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Lasers
Lasers (continued)
• The photons flash to and fro between the mirrors, becoming
amplified with each pass.
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Lasers
Lasers (continued)
• Some photons “leak” out of one of the mirrors, which is only
partially reflecting. These make up the laser beam.
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