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
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 Þ 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.
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.
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.
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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 Description Source
Continuous Continuous display of l Hot, dense like hot metal
Emission Only certain l present Excited (hot) gas
Absorption Certain l missing Radiation from hot, dense source passes through cool gas
Each element has a distinct set of emission and/or absorption lines.
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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
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Explains Emission Spectra:
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Explains Absorption Spectra:
Example of absorption spectrum