Astronomy Notes – Electromagnetic Spectrum, Types of Spectra
BRING:
Curtain
Gratings
Regular light bulb
Energy saver light bulb
H, He, Ne spectrum tubes and power supplies
Variac
Absorption liquids and light bulb
Slit (2)
Wood platforms
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
OBJECTIVES
What kind of electromagnetic radiation feels warm? Causes sunburn, skin cancer, and cataracts? Provides
doctors and dentists with “photos”? Warms food? Can you "see"? Penetrates smoke and dust?
How much of the universe can you see with your eyes, even with a telescope?
What is similar about these EM waves?
What is different about these EM waves?
Given a type of EM wave, give some idea of the energy of the source of that EM wave. For example, does
a radio wave come from a very energetic source or a medium energy source or a not so energetic source?
Given a type of EM wave, give some idea of the temperature of the source of that EM wave. For example,
does a radio wave come from a cool, a warm, or a hot source?
State the two kinds of EM waves that penetrate Earth’s atmosphere?
*Name and describe the three types of spectra.
*Describe a source or process that produces a continuous spectrum. An emission spectrum. An absorption
spectrum.
*Recognize that each element has a unique spectrum.
Define element.
Discuss how astronomers can tell what elements are in a gas cloud or in a star.
IN CLASS:
EM SPECTRUM
Visible
ROY G. BIV
 Long λ (700 nm)
Short λ (400 nm) 
Low f
High f
Low energy
High energy
<5% of universe
Part of the above from http://library.thinkquest.org/10380/advanced.shtml
Where have you encountered these waves?
Radio?
Microwave?
IR?
Visible? Duh
UV?
X-rays?
Gamma rays?
Since all 7 types of EM waves are the same thing, how are they similar?
All travel through the vacuum of space (sound cannot)
All travel in a vacuum at the speed of light
Other (for a different course, e.g. generated by accelerating charges)
How are they different?
Wavelength
Frequency
Energy of source
Temperature of source
Energy and EM waves
To detect a very energetic event like two neutron stars merging, “look” for this kind of wave:
1. Radio
2. Micro
3. IR
4. Visible
5. UV
6. X-ray
7. Gamma
To detect a very low energy event like the spin flip of a H atom, “look” for this kind of wave:
1. Radio
2. Micro
3. IR
4. Visible
5. UV
6. X-ray
7. Gamma
To detect the birth of a star in a gas cloud, “look” for this kind of wave:
1. Radio
2. Micro
3. IR
4. Visible
5. UV
6. X-ray
7. Gamma
What kinds of EM waves penetrate the Earth’s atmosphere?
From www.funkybiology.com
Side note: This is the issue with the “Ozone Hole”, a thinning of the Earth’s ozone layer allowing more
UV to penetrate.
Why worry about all the other wavelengths, why not just look with your eyes?
Look at the three questions under Energy and EM waves above. Can you answer it?
For many photos of the universe at different wavelengths
http://coolcosmos.ipac.caltech.edu/cosmic_classroom/multiwavelength_astronomy/multiwavelength_mus
eum/index.html
Also see http://www.jpl.nasa.gov/news/features.cfm?feature=573
or http://sirtf.caltech.edu/EPO/Messier/gallery.html
or http://www.seds.org/messier
3 TYPES OF SPECTRA (CONTINUOUS, EMISSION, ABSORPTION)
Prisms and diffraction gratings split light into the various wavelengths revealing a spectrum.
Light source
produces
various λs
Light beam of
those λs
Prism or
diffraction
grating
separates λs
Observer sees a
spectrum of
those λs
Eye sketch by
lilmizb3th.deviantart.com
Hand out diffraction gratings
1. Continuous spectrum
Ex: Spectrum of incandescent bulb
Pass around filament bulb
Describe: Rainbow like, All 
Source: Hot, dense object
Ex:
Filament, surface of sun (photosphere)
Light beam of all λs
Hot, dense
light source
produces all λs
Prism
separates
all λs
Observer sees a
spectrum of all λs,
namely a
continuous
spectrum
2. Emission spectrum
Ex: Spectrum of H gas tube
Note three lines (maybe four): red, turquoise, violet
Describe: Bright line
Source: Excited gas
Ex:
Gas discharge tube, gas clouds in outer space (Radio image of H in galaxy)
See http://www.jpl.nasa.gov/news/features.cfm?feature=573
Ex: Spectrum of He
Note: Must be a gas, Not H
Each element has a unique spectra. Different gases have different spectra
See http://blueox.uoregon.edu/~courses/BrauImages/Chap04/FG04_003.jpg
Hot gas
produces a
few λs
Light beam of
those few λs
Observer sees a
bright line
spectrum of those
few λs, namely an
emission spectrum
Prism
separates λs
3. Absorption spectrum
Ex: Use video camera to show spectrum of incandescent bulb through liquid
Describe: Missing colors or dark lines or bands, Continuous with certain  missing
Source: Hot dense object shining through a cooler gas cloud
Light beam
missing
those few λs
Hot, dense
light source
produces
all λs
Light beam
of all λs
A few λs are
absorbed in
gas cloud
Prism
separates
λs
Observer sees an
absorption spectrum,
most λs but missing
those few λs
absorbed by the gas
Summarizes all three
Uncertain source
Other resources for types of spectra
See http://rst.gsfc.nasa.gov/Sect20/A7.html
Or http://csep10.phys.utk.edu/astr162/lect/light/absorption.html
Or http://youtu.be/sVev5RsKXog (Parts 1 and 2)
Homework:
Make a flash card for each of the objectives.
Work the Types of Spectra Tutorial (emailed)
Revised 5 January 2016