Lecture 35
Chapter 30
Emission of Light
(Sources of Light)
17-Nov-10
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Production of Light
• Incandescent (hot) source (Stars; light bulb)
– Give off continuous spectrum (all frequencies) of
EM waves (electrons vibrate randomly)
– Inefficient source of visual light unless
temperature very high – most radiation is IR (heat)
• Electrons moving between energy states in atoms,
molecules, solids
– Fluorescent light
– Light-emitting diode (LED)
– Laser
Incandescence
Incandescence
• The frequency at which the most radiation is emitted
by a hot body is proportional to its Kelvin temperature
fmax ∝ T
• Radiation curve of brightness versus frequency for
emitted light.
Spectrum of Incandescent Source
Light Emission from Atomic Transitions
• Light emission can be
understood in terms of the
“planetary” model of the atom.
• Just as each element is
characterized by the number of
electrons that occupy the orbits
or shells surrounding its atomic
nucleus, each element also
possesses its own characteristic
pattern of electron shells, or
electron energy states.
• These states are found only at
certain definite energies. We
sometime call these allowed
states electron quantum states,
or Bohr orbits.
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Electron States in Atom – Bohr Orbits
Light Emission from Electron Transition
• Electrons excited to higher energy quantum state by heating
or passage of electric current; drop back
Photon Frequency & Energy
• Emission of photon as electron de-excites
• The frequency of an emitted photon ~ energylevel difference in de-exciting:
E = hf
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Photon Energy & Frequency
• The photon (quantum wave-packet of light) has an
energy = difference in electron state energies
• The photon frequency is related to its energy by
the Planck relation:
E = hf
where E = energy (in Joules); f = frequency (Hz);
h = Planck’s constant = 6.63 x 10-34 J/Hz
• For example, transitions to the 2nd Bohr orbit in
Hydrogen from higher orbits give photons with
frequencies in the visible range
Excitation
CHECK YOURSELF
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 YOURSELF
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
• Each component of color is focused at a definite
position according to frequency.
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Optical Spectrum of Hydrogen
Emission Spectra
Spectral lines emitted are characteristic of
element
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Emission Spectra
CHECK YOURSELF
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:
The spectra of light emitted by atoms are
considered to be the fingerprints of
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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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Absorption Spectrum of Hydrogen
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. UV light going
in comes out a visible light,
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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 YOURSELF
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 some 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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Fluorescent lamps
• UV emitted by excited gas strikes phosphor
material that emits white light.
• About 40% efficient for visible light.
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Compact Fluorescent Lamp (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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LED Lamps
Light-emitting diodes (LEDs)
• Monochromatic photons emitted
when electrons change energy
states in a solid
• Low voltage source powers
LED.
• Very efficient at producing
visible light -- about seven times
better than incandescent light
bulb.
• Light is monochromatic, but can
get white LED by combining
RGB LEDs or by using blue
LED + phosphorescent material
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Lamp Efficiency
• Incandescent (hot) source (Stars; light bulb)
– Only about 10% power used gives visual light –
most radiation is IR (heat) – 10 lumens/Watt
• Atomic transition sources
– Fluorescent light – 45-50 lumens/Watt
– Light-emitting diode (LED)–70 lumens/Watt
Lamp Efficiency
CHECK YOURSELF
Which lamp is more efficient for emitting visible light?
A.
B.
C.
D.
Incandescent lamp
Fluorescent lamp
Both the same for the same wattage
None of the above.
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Lamp Efficiency
CHECK YOUR ANSWER
Which lamp is more efficient for emitting visible light?
A.
B.
C.
D.
Incandescent lamp
Fluorescent lamp
Both the same for the same wattage
None of the above.
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Lasers & Coherent Light
Most light sources give incoherent light
• Incoherent light (many frequencies and out of
phase)
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Lasers & Coherent Light
LEDs and atomic transitions can give
monochromatic light
• Monochromatic light out of phase
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Lasers
Lasers can produce light that is monochromatic
AND coherent
• Coherent light of identical frequencies in phase
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Lasers
• 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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Key Points of Lecture 35
• Types of light sources
• Incandescent source
• Light emission from atomic transitions
• Emission Spectra
• Absorption Spectra
• Fluorescence
• Phosphorescence
• Lamps and Lamp Efficiency
• Lasers
z Before Monday Nov. 29, finish Hewitt Chap. 29.
z Homework #24 due by 11:00 PM Monday Nov. 29
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