Astronomy Rough Notes – Bohr Model of Atom, Lines in Spectra
BRING:
Curtain
Variac
Clear, single filament light bulb
Tennis balls
Box/chair to stand on
DISCLAIMER: These notes do NOT cover everything you need to know. You may need to look up some
item or concept online or in a text. Test questions are not exact copies of the OBJECTIVES but if you
know the OBJECTIVES thoroughly, you should do well on the exams.
HANDOUTS:
None
OTHER RESOURCES:
http://youtu.be/sVev5RsKXog (Parts 1 and 2)
OBJECTIVES
Describe the two major characteristics of black body radiation.
Draw the intensity vs wavelength graph for a black body.
What happens to that graph if the temperature of the body increases?
If you turn up the temperature of a light bulb, what did you notice about the peak wavelength and about
the overall radiation.
Draw two graphs of intensity vs wavelength for the low temperature bulb and the high temperature bulb.
Identify the three basic particles in an atom. Know whether their charge is positive or negative or neutral.
Know their approximate relative masses (not the mass itself, just how they compare). Know where each is
located in the atom.
Describe or sketch a simple model of the atom (the Bohr model) including permitted orbits.
Given a model of the atom, show which jumps correspond to emission spectra and which to absorption
spectra.
Given a model of the atom showing several energy levels, identify which photon comes from which
electron transition. (See the tutorial.)
What information can astronomers obtain from the spectrum of a star, galaxy or gas cloud?
What did Annie Cannon contribute to the study of spectra? Cecilia Payne?
State the two chief components of stars. What percent is each?
Define and differentiate between the following: atom, element, molecule
MATERIAL:
Black body radiation
Think of filament of light bulb
1. Billions and billions of atoms all vibrating at slightly different frequencies, emitting slightly
different λ
2. A few long λ, a few short λ but most λ near the middle.
3. Graph Intensity vs λ
I
n
t
e
n
s
i
t
y
λpeak
Light bulb demo
As T rises:
Color redder to bluer
Brighter
So as T rises:
More overall radiation (intensity rises)
Shorter peak λ ( λpeak )
For more on black body radiation, see
http://www.uwgb.edu/dutchs/CosmosNotes/spectra.htm (Nice picture)
Or http://www.spacegrant.montana.edu/msiproject/light.html
Or http://www.nrao.edu/index.php/learn/radioastronomy/radiowaves
Or http://www.uwgb.edu/dutchs/CosmosNotes/spectra.htm
For an interactive simulation, see http://phet.colorado.edu/en/simulation/blackbody-spectrum
Tutorial will be emailed to you. Be sure to work it.
SPECTRAL LINES OF SUN
Show spectral lines of sun
See http://coolcosmos.ipac.caltech.edu/cosmic_classroom/cosmic_reference/images/solarspectra.jpg
Absorption  hot dense photosphere with gas around
Note mix of lines/mix of elements
SPECTRA OF OTHER STARS
Each stripe of light is the spectrum of one star.
Note that each spectrum tapers off at the ends like the black body graphs above.
Not e the different lines in each spectrum for different stars.
From: Sky Publishing Corp, Cambridge Mass.
WHY SPECTRAL LINES
Why lines in spectra? Why different lines? Physics in late 1800s
Show stellar spectra/line patterns
O stars are hot, M stars are cool
Note how hot O stars show more in the shorter λ, cooler M stars show more in the longer λ
Annie Cannon at Harvard University was legendary at classifying stars. She looked at over 100 000 star
spectra and classified them by their relative temperatures.
See video “Ring of Truth” Part 6 of 6 “Doubt” First 12 minutes
Not available for free on the web. I have a copy in my office.
Structure of the atom - Bohr model and connection to spectra
http://www.colorado.edu/physics/2000/quantumzone/bohr.html
and http://csep10.phys.utk.edu/astr162/lect/light/bohr.html
Bohr model
Electrons in atoms can have only certain “orbits” or energies and not others.
Model the atom with electrons in orbit around the nucleus. Electrons can jump up or down between
energy levels but can never be in between.
Electrons can jump down
by emitting energy
Electron orbits
Nucleus containing
protons (+charge) and
neutrons (neutral)
Electrons can jump up if
they absorb energy
When an electron jumps down, it emits all its energy at once in a bundle called a photon. The bigger the
jump, the higher the energy of the photon. When an electron absorbs a photon of just the right energy, it
jumps up.
Example – A model atom is show below.
2
3
1
Question: Of the electron jumps shown, which jumps emit photons and which absorb photons?
Emit:
Absorb:
Question: Suppose you are told that a blue and a green photon are emitted and a red photon is absorbed.
Which jump corresponds with which?
Blue photon emitted
Green photon emitted
Red photon absorbed
Example tennis ball demo
Tutorial will be sent
Video: “Ring of Truth” again
Begin ~13:30 Quantum ladder
A word about the video
The video shows these orbits as a quantum ladder like the one shown below.
The ground state is the lowest energy the electron can have.
State 2
State 1
Ground state
--------------------------------------------------------------------------------------------------------------------Which photon has the
highest energy emitted
from the electron
transitions shown?
----------------------------------------------------------------------------------------------------------------------
1
2
3
Green, red, and violet photons are
emitted from the atom whose
quantum ladder is shown on the
left. Which color comes from
which transition? What color lines
are in the spectrum of this atom?
Continue video through Cecilia’s thesis to ~26:00
Also see http://www.colorado.edu/physics/2000/quantumzone/
Recipe of stars and universe
All that “glows” is 90% H and 10% He
See last page for summary of spectra and electron transitions.
Homework
Make a flashcard for each objective.
Read.
Work the tutorial on Blackbody radiation that was emailed.
Work the tutorial on “Light and Atoms” that was emailed.
Revised 5 January 2016
Astronomy – Brief Review of Spectra and Connection to the Atom
Observation:
3 Types
Continuous
Emission
Absorption
Description
Continuous display of 
Only certain  present
Certain  missing
Source
Hot, dense like hot metal
Excited (hot) gas
Radiation from hot, dense source passes through cool gas
Each element has a distinct set of emission and/or absorption lines.
*******************************************************************************
Model:
Atoms exist only with discrete energies. Atoms of each element have a distinct set of energy levels. Energy levels can be
represented as a quantum ladder.
Example
State 2
State 1
Energy
An electron (represented by a dot
) can be in state
2 or in state 1 or in the ground state but it cannot be in
between.
Ground state
*******************************************************************************
Explains Emission Spectra:
This electron transition down (small jump down) emits a low
energy photon (like red). Many of these jumps emit many
“red” photons that show up as one red line in the spectrum.
This electron transition down (big
jump down) emits a high energy
photon (like violet). Many of
these jumps down emit “violet”
photons that show up as one violet
line in the spectrum.
This electron transition down emits an intermediate energy
photon (like green). Many of these jumps down emit many
“green” photons that show up as one green line in the
spectrum.
********************************************************************************
Explains Absorption Spectra:
This electron transition up (small jump up) absorbs a
low energy photon (like red). Many of these jumps up
This electron transition up (big jump
show up as one missing red line in the spectrum.
up) absorbs a high energy photon (like
violet). Many of these jumps up absorb
“violet” photons that show up as one
missing violet line in the spectrum.
This electron transition up absorbs an intermediate
energy photon (like green). Many of these jumps up
absorb many “green” photons that show up as one
missing green line in the spectrum.
Example of absorption spectrum
Hot, dense surface
Emits photons of all 
(continuous spectrum)
All 
photons
H gas cloud
Absorbs certain
photons
All  photons
minus those
absorbed by H
Astronomer sees
absorption spectrum
(continuous but missing
H )