04Lab_Neon-Lights
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Montgomery College *
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101
Subject
Astronomy
Date
Dec 6, 2023
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docx
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Uploaded by bethelyared
M
ONTGOMERY
C
OLLEGE
– R
OCKVILLE
A
STRONOMY
101 ASTR101
Laboratory 4 – Neon Lights
Name: Bethel Yared
Spectroscopy is one of the most important tools an astronomer has. Spectra
can tell us about a star’s temperature, composition, and motion. In this lab
you are going to explore how spectra are formed by studying emission line
spectra..
PART A
Go to
http://phet.colorado.edu/en/simulation/discharge-lamps
Click the “Download” button and open the simulation.
There are two tabs at the top of the simulation:
One Atom
and
Multiple
Atoms
. For now, stay on the
One Atom
tab.
The figure shown in the sim is a glass discharge lamp. It’s basically a glass
container filled with a particular gas (hydrogen, sodium, or mercury, for
example) connected to a battery.
In this case there is one little hydrogen atom (labeled with the number “1”
inside the glass container. When the battery is on, electrons flow into the
glass container and can bump into the hydrogen atom.
Let’s observe what happens when an electron and hydrogen atom collide.
The higher the voltage of the battery, the more energetic the electrons will
be. Let’s start our electrons out at a low energy.
Slide the battery switch to 3.00 V. Select
Single
for
Electron Production
,
and then fire the electron.
What happens
to
the
electron?
Nothing much happened besides the electron running from the left to
the right rising, flowing through energy.
Let’s see what happens when there are more electrons. Select
Continuous
for
Electron Production
and move the slider up as much as you want. (If
the electrons aren’t firing, press the play (
) button at the bottom of the
screen.)
What happens
The electrons now continue to move one after the other from left to
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to
the
electrons
now?
right
What do you think you need to do
get the electrons to interact with the
hydrogen atom?
The voltage of the battery will need to be
increased.
Stop the electrons from firing with the pause (
) button on the bottom of the
sim. Turn the voltage on the battery to about 23.00 V, and then resume the
electrons firing with the play (
) button. The electrons should now interact
with the hydrogen.
The
Legend
in the upper right-hand corner of the screen can be used to
identify what is coming off the collision of the electrons with the hydrogen
atom.
What comes out of the collision?
(Select
Run in slow motion
under
Options
if you need to.)
Photons are coming out of the collision.
Now look at the energy level diagram of hydrogen shown on the right (under
the
Atom Type
). The diagram is showing in real time how the electrons are
interacting with the hydrogen atom. When an electron jumps from a higher
to a lower energy level, a photon is emitted, shown by a squiggly black line.
We want to look more closely at these photons. Select
Spectrometer
under
options. A spectrum (a plot of intensity vs. wavelength) should appear below
the discharge tube. The photons are being separated by energy. Uncheck the
Run in slow motion
box and watch the spectrum. Each time a purple
photon is emitted, the spectrum records one purple square. (For example, if
there are 10 purple photons emitted, then the purple line will be 10 squares
high. The spectrum is essentially a histogram of the emitted photons sorted
by wavelength.
Let the electrons run for a few minutes. You should see several lines
beginning to form.
Pause the electrons and reset the spectrometer. Slide the battery to 30.00 V
and the
Electron Production
to
100%
. Fire the electrons continuously for
at least two full minutes.
What color lines do you see building
-white
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in the spectrum?
-purple
-red
-navy blue
turquoise
What color appears around 410 nm
purple
If you run the simulation long enough, you will see four lines appear in the
visible portion of the spectrum (between 400 nm and 700 nm. Remember, a
‘nm’ is a nanometer, or 10
-9
m. It’s really small!) What you are seeing is
called an
emission line spectrum
- bright lines on a dark background.
The lines in the spectrum appear when the electron drops from a higher
energy level (also referred to as an excited state) to a lower energy level.
When the electron drops, it emits a photon of a specific energy. The energy
of the photon is the difference between the energy levels. (The lowest
energy level is referred to as the ground state.)
The four visible lines you see in hydrogen result from the electron dropping
from a higher energy level to the second energy level (level 2).These lines
are part of what is named the
Balmer series
.
Record the wavelengths you observe from the sim (in nm) for the four
visible
emission lines in the
Table 1
below under
Observed wavelength
.
Using the internet (or physics book if you have one!), look up the
wavelengths of the visible Balmer lines and record in
Table 1
under
Actual
wavelength
. They should be comparable. If they aren’t, look again at the
values you observed in the sim spectrometer. (Google “Balmer Series”. The
Wikipedia article will have a table with the lines.)
Table 1
Color
Observed wavelength
(nm)
Actual wavelength
(nm)
Violet
410nm
410nm
Blue
435nm
434nm
Aqua
485nm
486nm
Red
655nm
656nm
Which visible line has the highest
energy?
Red
Which visible line has the lowest
Violet
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energy?
Will a hydrogen atom emit
light if the atom’s electron
is in the ground state?
Explain your reasoning.
No, the hydrogen atom will not emit light.
This is because it’s already at ground state,
there is no lower energy level to move.
Since it would have to move from a higher
energy level to a low energy level in order
to emit light.
The figure below shows the energy level diagram for a hydrogen atom. (The
‘eV’ stands for a unit of energy called an electron volt.) The dots represent
electrons, and the arrows (labeled A-F) represent the electron’s “dropping” to
a lower energy level.
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Match visible spectral lines you observed in the simulation with the transition
in the Figure. The first one has been completed for you. (Remember that the
visible lines are part of the Balmer series that end on level 2.)
Violet
B
Blue
A
Aqua
D
Red
E
PART B
Select the
Multiple Atoms
tab. You should now see many hydrogen atoms
in glass container.
Make sure the
Spectrometer
option is selected. Set the battery voltage to
30.00 V and continuous electron production at 100%. The hydrogen emission
lines should show up very quickly!
Now, what happens if we put a different element into our gas discharge
tube?
Pause the simulation. From the
Atom Type
menu, select
Neon
. Run
continuous electron production at 100% for at least thirty seconds.
What colors are the emission lines
you see?
There are no emission lines.
You’ve probably seen neon lights on restaurant or store fronts. A true neon
light will contain a gas tube of neon atoms.
When we look at a neon light, our eyes blend together all the emission lines
(our eyes, unfortunately, do not have spectrometers installed!). The color we
see is the combination of the emission lines.
What color do you think a neon light
would appear to your eye?
I would assume it would be almost a
red color.
Keep the simulation running and select “Sodium” from the “Atom Type”
menu.
What color do you think a sodium
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light would appear to your eye?
I would say the warm orangish yellow
color.
Keep the simulation running and select “Mercury” from the “Atom Type”
menu.
What color do you think a mercury
light would appear to your eye?
(Hint: which
visible
line grows the
fastest?)
It would appear as more of a white
grey color.
Click on the “View Picture of Discharge Lamps” to test your predictions for
the colors of neon, sodium and mercury lights.
Were your predictions correct?
My first two predictions were correct,
but I messed up on the color for the
mercury light.
Sodium and mercury are commonly used for street lights. Next time you are
out driving at night, look at the colors of the lights. You may be able to
determine the type of light it is by the color!
PART C
Please respond to the following in your own words.
How is an emission line spectrum formed?
For an emission line spectrum to form an atom or molecule is in a higher
excited state but then returns to its lower energy levels causing this source
to emit a particular radiation wavelength.
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Use your knowledge of atoms to explain why different gas discharge lamps
are different colors.
Different gas discharge lamps appear different colors because of the
different wavelengths of light each element emits. To return to their ground
state each element will emit different wavelengths causing the different
discharge lamps to be various colors.
Relate what is seen in a spectrometer to what is physically happening.
The spectrometer showing the nanometers adding up in a line as the
wavelength continue to increase as the electrons pass through the hydrogen
atom. This visual is exemplify how the spectrometer measures absorbance,
transmittance, and the intensity of light. This is measuring the wavelengths
and light frequency that allows us to identify and analyze the split of the light
wave into component colors.
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