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Reminders
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Office hours:
1-2pm, July 18 (today!)
Watanabe Room 403
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Review Session tomorrow
Exam on Friday, 50 multiple choice questions, 75 minutes
Today:
Emission/Absorption Lines and the causes
Redshift and Blueshift of light
Learning from light: Spectra
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Spectrum: different amounts of
light from an object at different
wavelengths
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Thermal Blackbody - one type of
continuous spectrum
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The shape of spectrum depends
on temperature only
Learning from light: Spectra
Continuous
Emission
line
Absorption
line
Learning from light: Spectra
Spectrum: different amounts of light from an object at
different wavelengths
A white light bulb will show emission at many
wavelengths: continuous spectrum
A sodium bulb (filled with sodium gas) will emit light
only at very specific wavelengths: emission line
spectrum
If we put a cloud of sodium gas between us and the
white bulb, we will see most colors but some specific
wavelengths will be missing: absorption line spectrum
Emission line spectrum
Argon
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We see very specific colors of
light emitted from a low density
cloud of gas. We call these
features emission lines.
Lines we see depend on
compositions, density and
temperature
Hydrogen
Helium
Neon
Absorption line spectrum
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Dark parts of continuous
spectrum (“missing” light) are
absorption lines.
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Depend on composition,
density
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For a given element, absorption
and emission lines are at same
wavelengths
What cause Em/Abs lines?
Basic Atomic Physics
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Basic chemical elements (hydrogen, oxygen, carbon...) are
made up of unique combinations of:
Protons
Neutrons
Electrons
Basic Atomic Physics
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Atoms are TINY: 10-10 meters!
How many atoms do you think there are in one drop of
water?
1021 = 1,000,000,000,000,000,000,000 atoms
2/3 hydrogen, 1/3 oxygen
Basic Atomic Physics
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Electrons can only appear at
certain orbits (carrying
certain energies) in the
atom --- Energy levels in
atoms
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The configuration of orbits
depend on elements (e.g.
Hydrogen and Helium are
different)
Basic Atomic Physics
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Energy levels in atoms are
quantized
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Strange but true: electrons
in atoms can only have
specific amount of energy
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Like rungs on a ladder
Electrons can go up and
down only between these
levels
Basic Atomic Physics
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Lowest energy level of an
electron is called its ground
state
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Energies of electrons in unit
called electron-Volts, or eV
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Ground state has energy of
0 eV
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Higher levels are called
excited states
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Each excited state requires
a specific energy to get to
Basic Atomic Physics
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How do these electrons go up and down at
these orbits?
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Recall: light has property of particle, and we
call them as photons
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A photon carries a particular amount of
energy that depends on its frequency
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The higher the frequency of the photon, the
more energy it carries
The electron can go up and down at the orbit with emit or
absorb photons (carrying specific amount of energy)
Energy states in Hydrogen
Class Action!
Which atom would be absorbing light with the greatest energy?
Class Action!
Which atom would emit light with the longest wavelength?
Questions?
What causes em/abs lines?
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As atoms bump into each other,
they can transfer energy
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If bump is of just the right
energy, it can cause an electron
to jump between energy states
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Electron jumps back to lower
state by getting rid of its newly
acquired energy
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Emit photon (light) with the
exact amount of energy to
“relax” from excited state
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Energy must be conserved!
Why are these lines useful?
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Every atom has its own
specific set of possible
transitions
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So each emits and absorbs
at different specific
wavelengths
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Creating unique spectra we
can use as fingerprints to
identify what something is
made of!
Argon
Hydrogen
Helium
Neon
What causes emission lines?
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As long as gas is warm enough,
the atoms bump into each other
often
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Continuously exciting electrons
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Creates emission line spectrum
That give off photons as they
relax
As gas gets hotter, bumps have
more energy
Emission line spectrum
Continuous
Emission
line
Absorption
line
What causes absorption lines?
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Illuminate a cold gas (most atoms
in ground state)
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Atoms can absorb photons of
just the right energy to excite
electrons
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The light at these energies
(wavelengths) does not make it
to the observer
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When electrons relax, photons
emitted in all directions, not just
along line of sight, so the
absorption lines don’t all fill in
Absorption line spectrum
Continuous
Emission
line
Absorption
line
Class Action!
Class Action!
Questions?
Learning more from light
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We’ve seen how to find out what makes up a star or galaxy by
using emission/absorption lines
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How hot something is based on its color
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What else can we learn?
Based on the Blackbody radiation, we can learn the sizes and
temperatures of stars
Learning more from light
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How fast something is moving towards or away from us
Using a “light” version of the Doppler effect
We are familiar with this effect for sound...
The Doppler effect
Doppler effect for light
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If something is moving
towards us, the emitted light
wave peaks are closer to
each other than if it was
standing still
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The wavelength of light we
observe will be shifted to
shorter wavelengths, which
we call a blueshift
Doppler effect for light
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If something is moving away from
us, the emitted light wave peaks are
further apart than if it was standing
still
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The wavelength of light we observe
will be shifted to longer
wavelengths, which we call a
redshift
Measuring redshift
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What kind of spectrum
has features at specific
wavelengths that we can
use to measure the red/
blueshift of an object
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Emission or Absorption
What redshift doesn’t tell us..
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Doppler shift only tells
us about component of
object’s motion toward
or away from us
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We call this the radial
direction
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It tells us nothing about
the motion in other
directions
Class Action!
Class Action!
Questions?
Lecture Tutorial
Break up into groups of 2-3
NO MORE THAN THREE
In your group, work through the following:
Doppler Shift (pages 73 - 77)
Discuss the answers – don’t be silent!
Nick and I will be roaming around if you need
help…