Moon Project
• Because we have had so few clear nights, the
Moon project deadline is extended one week until
February 25.
• Locate the Moon with respect to the STARS on
the star map (NOT the horizon)
– You need to find star patterns you recognize in the sky
and on the map to determine where the Moon is
– Depending on the time of night you look the Moon will
be in different parts of the sky, but on a single night
always close to the same stars.
Announcements
• Exams will be returned Wednesday. Scoring
office results did not come until late today and
there are some corrections needed.
• Future assignments will be graded, but you are
encouraged to take them until you get a perfect
score. We record your last attempt.
• Assignment for Wednesday is in two parts:
– Part A: Astronomy Place tutorial “Light &
Spectroscopy” done on Astronomy Place
– Part B: Planet assignment 1 on Angel
Objectives
• Describe the process of emission and
absorption of photons by atoms
• Relate the photon energy and wavelength to
the temperature of the source
• Explain why each element’s atoms emit &
absorb photons with only certain colors,
energies, wavelengths
Emission & Absorption of Light
• The most common confusion mentioned
was how different elements emit & absorb
different color light
• The second most common confusion was
about thermal radiation
Today we will discuss these two issues.
MOST IMPORTANT
• Electrons Emit & Absorb Photons of Light
• Energy is CONSERVED
– When an electron emits a photon, the
electron loses the energy the photon
carries off
– When an electron absorbs a photon,
the electron gain the energy the
photon had
Temperature
• Measures motion of atoms (& molecules,
ions, nuclei, electrons)
• Hotter = moving Faster
kT = 1/2 m v2
k=number
m=mass
T=temperature
v=speed
Not a required equation
How do Atoms gain Energy
1. Absorbing a Photon
2. Colliding with another atom, electron or ion
•
•
Hot gas
Electric current (demo)
Thermal Radiation
1. Hotter -> Brighter
emit more radiation per unit area.
 Stephan-Boltzmann Law
4
 E = T
2. Hotter -> Bluer
emit bluer (higher energy) photons
 Wien Law
 max = 2.9 x 106 / T(K) [nm]
3. Bigger -> Brighter
(more photons)
Thermal Radiation
Atom
electron
nucleus
p+n
proton
neutron
e-
Energy Levels
Atoms are not the same as planetary systems
Only certain Orbits (Energy Levels) are
allowed an electron in an atom.
Like an elevator - can stop only at the
floors, not in between
Energy Levels
E
n
e
r
g
y
E3
E2
E1
Absorption: e- in lower energy orbit
absorbs a photon
& jumps to higher energy orbit
Emission: e- in higher energy orbit
jumps to lower energy orbit
& emits a photon
Absorption & Emission of Light
Interaction of Light with Matter
Hydrogen
• Remember that each
electron is only allowed to
have certain energies in an
atom.
• Electrons can absorb light
and gain energy or emit
light when they lose energy.
• It is easiest to think of light as a photon when discussing
its interaction with matter.
• Only photons whose energies (colors) match the “jump”
in electron energy levels can be emitted or absorbed.
Light as Information Bearer
We can separate light into its different wavelengths (spectrum).
By studying the spectrum of an object, we can learn its:
1 Composition
2 Temperature
3 Velocity
Emission Spectra
• The atoms of each
element have their
own distinctive set of
electron energy
levels.
• Each element emits
its own pattern of
colors, like
fingerprints.
• If it is a hot gas, we
see only these colors,
called an emission
line spectrum.