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Goal: To understand how light is
used to understand what
humankind has discovered about
astronomy.
Objectives:
1) To learn the Properties of light
2) To understand how the 3
different types of spectra are
created.
3) Play the GUESS THE GAS
CONTEST!!!
Properties of light
• Light can act like either a particle or a
wave. Most commonly acts like a wave.
• Like waves you are familiar with, light
waves have a wavelength and energy.
• Energy = a constant * speed of light / wavelength
Which of the following would NOT
emit light waves?
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Dentist X-ray machine
An outgoing radio tower
Radar machine
The night side of the moon
Neon sign
All of the above emit light
None of these emit light
Spectrum of Electromagnetic Radiation
Region
Wavelength
(Angstroms)
Wavelength
(centimeters)
Frequency
(Hz)
Energy
(eV)
Radio
> 109
> 10
< 3 x 109
< 10-5
Microwave
109 - 106
10 - 0.01
3 x 109 - 3 x 1012
10-5 - 0.01
Infrared
106 - 7000
0.01 - 7 x 10-5
3 x 1012 - 4.3 x 1014
0.01 - 2
Visible
7000 - 4000
Ultraviolet
4000 - 10
4 x 10-5 - 10-7
7.5 x 1014 - 3 x 1017
3 - 103
X-Rays
10 - 0.1
10-7 - 10-9
3 x 1017 - 3 x 1019
103 - 105
Gamma Rays
< 0.1
< 10-9
> 3 x 1019
> 105
7 x 10-5 - 4 x 10-5 4.3 x 1014 - 7.5 x 1014
2-3
A radio wave is sent out from
planet earth. How long will it take
that radio wave to travel to Alpha
Centauri B (4.3 light years away)
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A) a little over 1 year
B) a little over 2 years
C) a little over 4 years
D) a little over 8 years
E) this radio wave can’t possibly get there.
How is light created?
• Light is created by electrons changing
energy states – to conserve energy.
• So, to understand light, you have to
understand the atom.
• Atoms are made of a nucleus (with +
charged protons and uncharged neutrons)
and shells of – charged electrons.
• Much like different distances from an object
creates a difference gravitational energy, so
to does the distance from a nucleus.
• However, there are only certain specific
allowed energy states in an atom.
Spectra, meet the spectra!
• There are 3 types of spectra:
• 1) continuous – a smooth light curve with
no breaks and light at all wavelengths
• 2) absorption – a continuum which has
parts taken out (kind of like a hockey
goalie).
• 3) emission – either a continuum with
emission added to it, or just a series of
special lines.
Condescend Light Bulb
• Take the spectral glasses and take a look
at the Condescend light bulb. What type
of spectrum would you classify this as?
• A) Continuous
• B) Absorption
• C) Emission
Blackbody Spectrum – continuous
• All objects which absorb most of the light
which passes through them emit energy
as a “blackbody”. The shape of the
blackbody spectrum is always the same.
• The strength of the spectrum (i.e. how
much light it emits) and the peak
wavelength of the spectrum depend on the
temperature of the object.
http://theory.uwinnipeg.ca/users/gabor/foundations/quantum/slide6.html
Windex
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What type of spectrum does this produce?
A) Continuous
B) Absorption
C) Emission
Absorption Spectrum
• As you can see, the Windex creates a
spectrum where you have a continuous
portion, but a huge chunk is taken out.
• This chunk is absorbed by the Windex.
• The Windex, not surprising, is blue
because of the light it absorbs.
Absorption spectra in Astronomy
• Windex is a solution of very complex molecules
though, and in Astronomy most Absorption
spectra are created by gasses of a few atoms
and few molecules.
• Therefore, most Absorption spectra in
Astronomy consist of thin dark lines, or thin
areas where most of the energy is removed at
just a few wavelengths.
• Any “thin” substance which is in front of a
continuous light source will produce an
absorption spectrum IF it is cooler than the
continuous source.
Absorption – our use
• When you find an emission or absorption
spectrum, you know what the makeup of
the gas or material that emitted it.
• Each element and each molecule have
their own specific energies they can emit.
This is a fingerprint of the gas.
• This is why we know the sun is made of
mostly H and He.
Fluorescent lights
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What type of spectrum does this produce?
A) Continuous
B) Absorption
C) Emission
Fluorescent lights
• This is emission.
• The electrons in the gas get “excited” by the
energy put through the gas.
• The electrons then “cascade” down to emit
several photons of light. One is a optical photon.
• There are different specific energy states, and
the lines are produced by the different
combinations of energy states (starting/ending).
• Any thin gas will produce an emission spectrum
if you JUST look at the gas.
• Any thin gas which is HOTTER than a
background continuous source will ALSO
produce emission features!
Guess the gas contest!
• For each of the following tubes you may
find it helpful to draw the pattern of light.
You will guess which gas is in each tube.
• Anyone who can get all of them gets a
bonus 5 points!
• Everyone who turns in with guesses and
patterns on all tubes gets the normal
points.
• If you can get all but 1 I will give you a
bonus 3 points.
• If you get 4 of 6 you get 1 bonus point.
Conclusion
• We examined the properties of light.
• We discovered the method of creating
light.
• We discovered the different spectrum and
explored how to create each one of them
(and therefore what we can use them for).
• We looked at some gasses and found
what most of them were from the light they
emitted.