Chapter 30
Light Emission
Excitation and de-excitation of
electrons
Simple model of the atom
Electron absorbs some energy which
causes it to “jump” to a higher energy
level. This is called excitation.
E2
Energy
E1
Nucleus
de-excitation
The electron, preferring to be in the
lower energy, immediately drops back
down to the lower energy level.
E2
E1
Energy
In order to conserve energy, a photon
(discrete bundle of energy) is emitted.
E2
E1
Photon Energy
Photon Energy = E2 – E1
Because of the behavior of matter and energy at
the atomic scale, which is governed by Quantum
Mechanics, the energy of the photon is equal to
the difference in the energies of the two energy
levels and is also proportional to its frequency.
That is: Ephoton = hf where
•h = Planck’s constant.
•f = frequency of the photon.
Depending on the element, only certain
excitations and de-excitations (or transitions) are
allowed for the electrons.
•The cumulative result is that an atom of
a particular element will only have light
emissions of certain frequencies.
•The combination of all the allowed
transitions produces an emission
spectrum for that particular element.
•No two elements have the same
emission spectrum, so the emission
spectrum can be used to identify the
element in question.
Emission Spectrum
(Chaisson/McMillan) Astronomy
Emission Lines for the Hydrogen Atom
(Chaisson/McMillan) Astronomy
Emission Spectra of Some Common Elements
(Chaisson/McMillan) Astronomy
Emission Spectrum
•Emission spectra are emitted
by atoms in a gaseous state
where the atoms are so far apart
that interactions between them
are negligible.
i.e. (each atom behaves as an
isolated system.)
Incandescence
• Hot Matter in condensed states
(solid or liquid or dense gas) nearly
always emits radiation with a
continuous spectrum.
•This is called Incandescence.
Continuous Spectrum
•This results in a much larger number of
possible transitions with corresponding
frequencies of radiant energy, producing
a continuous spectrum.
•When the atoms are in a condensed state,
the electrons can make transitions not only
within the energy levels of their own atom,
but also between the levels of neighboring
atoms.
Continuous Spectrum
(Chaisson/McMillan) Astronomy
The continuous spectrum emitted by an ideal
surface that is completely nonreflecting is
called blackbody radiation.
•Example: A Star.
•The predominant frequency of radiation
(the peak frequency) is proportional to the
temperature of the emitter:
•f  Temperature
Blackbody (Planck) Curves
(Chaisson/McMillan) Astronomy
Absorption Spectrum
• When we view light from an incandescent
source through a spectroscope, we see a
continuous spectrum.
(Chaisson/McMillan) Astronomy
If a gas is placed between the source and the
spectroscope, we see an absorption spectrum the inverse of emission spectrum.
(Chaisson/McMillan) Astronomy
Example Emission and
Absorption Spectra of Sodium
(Chaisson/McMillan) Astronomy
Fluorescence
• Fluorescence occurs when UV light is
absorbed by the electrons of an atom and
visible light is emitted upon de-excitation.
Excitation
E2
UV light
E1
Nucleus
De-excitation to intermediate level
E2
E1
Nucleus
Visible
Light
Fluorescent Lamps
High-speed
electron
e-
UV
Photon
Hg
atom
Phosphor
Coating
Fluorescence
Phosphorescence
• Atoms or molecules are excited by incident
visible light and remain in an excited state
for a long time before they de-excite.
• This allows phosphorescent materials to
glow in the dark.
Phosphorescent Dove
Glow in the Dark Dove
Incoherent and Coherent light
White light from an
incandescent light bulb
is called incoherent
light.
The light is composed of different
colors and the waves are out of
phase with each other.
If we pass the white light through a filter, we
could get only one color (monochromatic)
light which would still be out of phase.
(incoherent red light)
Monochromatic, Coherent light
•By getting all the red light in phase, we
would have monochromatic, coherent light.
• This is the kind of light emitted by a
LASER.
Monochromatic, Coherent Red light
Lasers
by
of
Light
Amplification
Stimulated
Emission
Radiation
•High Voltage is applied to electrodes.
•Electrical discharge excites He atoms.
•Excited He atoms transfer energy to Ne
atoms via collisions.
•Neon atoms can stay in a prolonged excited
state (“metastable state”)
•Eventually, there are more neon atoms in the
metastable state than in lower energy states.
A population inversion now exists.
•As some neon atoms de-excite from the
metastable state, red photons are emitted.
As more neon atoms de-excite from the
metastable state, more red photons are
emitted.
• As these red photons pass other excited
neon atoms, they too are stimulated into
emitting a red photon of the exact same
frequency and in phase with stimulating
radiation.
•By placing mirrors on each end of the cavity
or tube, the light emission is amplified.
• With one end having a partially reflective
mirror, eventually the monochromatic,
coherent light is emitted from the LASER.
LASER DEMO
End of Chapter 30