How Matter Emits
Light: 2. Line
Emission and
Spectroscopy
Announcements
q  Homework # 3 is due today, Oct 20th.
q  Homework # 4 starts today, Thursday
October 20th, and is due Thursday October
27th
Assigned Reading
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Units 24 and 25
Today’s Goals
q Discuss and understand the other
mechanism, other than Blackbody, through
which matter can emit light
q  Familiarize ourselves with the concept of
atomic spectra
How do objects make light?
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There are two principal mechanisms
for producing electromagnetic
radiation
•  Blackbody radiation
• Spectral line emission of atoms and
molecules
Both of these mechanisms result from
accelerating/decelerating electrons! I.e., you accelerate
an electric charge to create EM radiation
How Atoms Emit Light:
absorption
§ The electrons in the
atoms first need to
absorb energy, either
from light (photons) or
from the collisions of
atoms with each other.
§ Then the electrons can
re-emit the absorbed
emission
energy.
§  I.e., they accelerate or
decelerate in their
orbits
Excitation of Atoms
(The atoms do not produce
light, yet)
v To change its energy levels,
an electron must either
absorb or emit a photon that
has the same amount of energy
as the difference between the
energy levels
v E1-E4 = Eph = hν = h c/λ
v Larger energy difference
means higher frequency.
v Different jumps in energy
levels means different
frequencies of light absorbed.
What Happens after the Excitation:
absorption
emission
Now they produce light
§ The excited electrons will
tend to go back to their
ground-state (state of
minimal energy)
§  They may choose a path
that causes multiple
`jumps’ through energy
levels of decreasing energy
§  Each `jump’ produces a
photon with energy:
E4-E3 = Eph = hν = h c/λ
§  When there are many
identical atoms, light with
the same wavelength (single
color or line) will be
produced
Do we see this effect?
Spectral Line Emission (photon
absorption)
If a photon of exactly
the right energy is
absorbed by an
electron in an atom,
the electron will gain
the energy of the
photon and jump to an
outer, higher energy
orbit.
A photon of the same energy is emitted when the
electron falls back down to its original orbit.
Spectral Line Emission
(collision)
Collisions (like in a
hot gas) can also
provide electrons
with enough
energy to change
energy levels.
A photon of the same energy is emitted
when the electron falls back down to its
original orbit.
Energy Levels in an Atom or
Molecule are Unique!
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The allowed electron energies are specific to
the kind of atom (element) or molecule.
The amount of energy in the electron’s orbit
is related to the average distance of the
electron from the nucleus. If we add enough
energy, the electron escapes all
together from the atom. The atom is then
ionized. Also the ionization energy is specific
to the atom or molecule.
Spectral Lines of Some Elements
Argon
Helium
Mercury
Sodium
Neon
Spectral lines are like a cosmic barcode system for elements.
Atoms of different elements have unique
spectral lines because each element
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has atoms of a unique color
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has a unique set of neutrons
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has a unique set of electron orbits
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has unique photons
Light bulb =
Blackbody radiation
The Solar Spectrum
There are similar absorption lines in the other regions of
the electromagnetic spectrum. Each line exactly
corresponds to chemical elements in the stars. The Sun (a star) is as a blackbody, and the gas in its
atmosphere (around it) is made of atoms that absorb its
light, and produce dark lines in its spectrum
Energy Levels of a Hydrogen Atom
Different allowed
“orbits” or energy
levels in a hydrogen
atom.
Emission line spectrum
Absorption line spectrum
Emission nebula
Reflection nebula
Decomposing an Object’s Spectral Features
Visible Spectrum of the Planet Mars
As seen through a prism
Encoded in an object’s spectrum is information about the emitter/
absorber. This is how we learn what the Universe is made of!
Spectra of Galaxies
Name that Spectrum!
A
C
Absorption
Emission
B
Continuous
Survey Question
A gas cloud that has become completely
ionized
1)
2)
3)
4)
does not have spectral line absorption
does not have spectral line emission
emits only x-rays
emits only radio waves
Survey Question
The number of electrons lost by an atom
in a gas (that is, it’s ionization state)
depends primarily on
1)
2)
3)
4)
the
the
the
the
velocity of the gas
temperature of the gas
level of the ground state
size of the gas cloud
The Doppler Effect:
other information contained in spectrum
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A moving light or sound source
emits a different frequency in the
forward direction than in the reverse
direction.
Think of the approaching (higher
pitch) and then receding (lower
pitch) police car to figure out how
this works.
In general …
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The “native” frequency at which an object is
emitting is called the rest frequency.
You will see/hear frequencies higher than the
rest frequency from objects moving towards you.
You will see/hear frequencies lower than the
rest frequency from objects moving away from
you.
Doppler Shift
The first crest travels out in
circle from the original position
of the plane
Shorter wavelength
(bluer)
At a later time, a second
crest is emitted from the
planes new position,
but the old crest keeps
moving out in a circle from
the planes original
position
The same thing happens again at
a later time
Longer
wavelength
(redder)
What we actually see when we do
spectroscopy
Emission spectrum of hot gas as seen in lab
Emission spectrum of hot gas as seen in rapidly
moving object
Is this object moving towards or away from us?
What we do with the Doppler
Shift
We derive the (approaching or receding) velocity
of the source.
q  The Doppler shift causes the rest wavelength, λr ,
emitted by an object to become bluer (blue-shift) or
redder (red-shift): λo- λr
q  To derive a velocity, we divide by a time T= 1/νr:
v = (λo- λr)/T = (λo- λr) νr = c (λo- λr)/ λr
v/c = (λo- λr)/ λr
This is the formula used to measured the
recession of galaxies in the Universe!
The Doppler shift
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An object shining red light with λ=656.3 nm is
moving at V=5,000,000 m/s away from you.
What is the color of the light that you see?
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v/c = (λo- λr)/λr
v/c =5x106/3x108 = 1.67x10-2 = (λo- λr)/λr
λo = λrx(1+1.67x10-2) = 667.3 nm
Survey Question
Two identical stars are observed from the Earth. Star
A’s emission lines (that are at visible wavelengths in
the rest frame) are observed to be at ultraviolet
wavelengths. The same emission lines for Star B are
observed to be at X-ray wavelengths. From these
observations you conclude that:
1) both stars are moving away from the Earth
2) Star A is moving towards the Earth faster than
Star B
3) Star B is moving towards the Earth faster than
Star A
4) Star B is moving away from the Earth while
Star A is moving towards the Earth.