Download Spectra Student Guide

Spectra Student Guide
Introduction:
In this lab you’ll use a high quality spectrometer, made by Project STAR, to examine
the spectra of a variety of light sources. The main goals are to practice accurately
observing and recording the appearance of a spectrum, and to think about the
different types of spectra and the characteristics of the objects that create them.
Background material:
The spectrum of a light source shows how the light intensity varies with the
wavelength of light. Basically, a spectrum records how much light is produced at each
color. There are three main categories of spectrum, which are produced in the
following three situations:
1. A hot opaque body, such as a dense gas
or solid, produces a continuous spectrum –
a complete rainbow of colors. The intensity
varies smoothly with wavelength.
2. A hot, low-density gas produces an
emission line spectrum – a series of bright
spectral lines against a dark background.
Light is only emitted at specific
wavelengths.
3. A cool, low-density gas in front of a hot
opaque body produces an absorption line
spectrum - similar to a continuous
spectrum, except with dark lines (dips in
intensity) at specific wavelengths.
Emission and absorption lines have a characteristic pattern that is determined by the
composition of the gas involved. For a given type of gas, the bright lines in an emission
spectrum, where the hot gas emits light, occur at exactly the same wavelengths as the
dark lines in the absorption spectrum, where the cool gas absorbs some of the
continuous spectrum’s light.
By measuring the spectrum of stars and nebula, and comparing them to spectra
observed in labs on Earth, astronomers are able to learn about the temperature and
composition of distant objects.
Review the background material on light and spectra:
http://astro.unl.edu/naap/hydrogen/light.html
http://astro.unl.edu/naap/blackbody/spectra.html
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A Project STAR spectrometer and various spectral tubes.
Using the Spectrometer
Using the Spectrometer:
Ca
lib
rat
ed
Sca
le
Eye Hole
Source
Figure 1: The STAR Spectrometer. Note the locations of the eye hole, the calibrated
scale that you look at through the eye hole and the position of the source with respect
The
Spectrometer.
the locations
of the eye hole, the calibrated scale
to STAR
the spectrometer.
This isNote
explained
in detail below.
Figure 1:
that you
ook at through the eye hole and the position of the source with respect to the spectrometer. This
the spectrometer
so that you can look through the grating in the narrow end. You
s explained Hold
in detail
below.
should be able to see two rows of calibration marks and numbers. Pay attention to the
lower row, which gives the wavelength (in nanometers, or nm) of the light in the
spectrometer
spectra aboveso
it. that you can look through the grating in the narrow end.
Hold the
You should
be able to see two rows of calibration marks and numbers. Pay attention to the lower row, which
To observe a spectrum, keep holding the spectrometer up to your eye, and turn your
gives the wavelength
nanometers,
nm) of the
theisspectra
it.
whole body(in
until
the slit at theor
right-hand
sidelight
of thein
front
pointedabove
at the source
of
you want to examine. (This is the most counter-intuitive part of the whole
To observe alight
spectrum,
keep holding the spectrometer up to your eye, and turn your whole body
procedure. Most people are tempted to just aim the middle of the spectrometer at the
until the slit light
at the
right-hand
side
of the
frontbeislined
pointed
at the
of light
you
source.
The light
source
should
up with
thesource
right side.)
When
youwant
have to examine.
the spectrometer
aimed properly,
spectrum
of procedure.
the light source
should
appear
above
This is the most
counter-intuitive
part ofa the
whole
Most
people
are
tempted to just
the wavelength scale.
aim the middle of the spectrometer at the light source. Aim the right side instead.) When you
have the spectrometer
aimed
a spectrum
ofhelp,
the ask
light
source
This procedure
takes properly,
a little practice.
If you need
your
TA. should appear above the
wavelength scale.
Observing Spectra
This procedure takes a little practice. If you need help, ask your TA.
Your TA will set up a variety of light sources for you to study in the lab, similar to the
ones we saw in class. We will use:
1
• Two light bulbs with different brightnesses.
• An emission tube of hydrogen gas.
• Two emission tubes labeled “Source A” and “Source B.”
• A white fluorescent light (i.e. a regular white strip light).
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In this lab, you will be asked to make observations of the spectra of these different
sources and answer questions about them. You can make the observations of the
spectra in any order you like (it will help to prevent crowding if people do these in
different orders). You can answer the questions at any time during the lab, but you
should make sure you have observed all the spectra before you leave the lab. Observe
each source with your eyes and through the STAR spectrometer and answer the
questions below.
Incandescent Light Bulbs.
In part one of this lab, we will study a common blackbody in everyday use: a light bulb.
You will observe incandescent light bulbs at two different brightnesses (which
correspond to two different temperatures).
Start by observing either light bulb:
1.
What type of spectrum do you see when you look at a light bulb through the
spectrometer?
2.
Where in the light bulb does the light come from? Describe the nature of this
source of light.
Bright light bulb:
3.
What color does the lightbulb appear?
4.
What is the smallest wavelength of light you can see when you view the bright
light bulb through a spectrometer (in nanometers) and what is it color?
5.
What is the longest wavelength of light you can see when you view this source
through a spectrometer (in nanometers) and what is it color?
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Dim light bulb:
6.
What color does the lightbulb appear?
7.
What is the smallest wavelength of light you can see when you view this source
through a spectrometer (in nanometers) and what is it color?
8.
What is the longest wavelength of light you can see when you view this source
through a spectrometer (in nanometers) and what is it color?
Comparing light bulb observations:
9.
Describe the differences between the two light bulb spectra. Specifically, how do
they differ in the relative amount of light produced at different wavelengths?
Plotting spectra:
10
The wavelengths of light which are
visible, and the regions that appear
blue and red, are also marked.
2
intensity of light Harbitrary unitsL
For a hot dense source, the amount
of light produced at different
wavelengths depends on the
temperature. This is difficult to
judge by eye, so we have plotted the
intensity of light versus wavelength
for a hot dense light source on a
bright (A) and a dim (B) setting in
the plot to the right.
Aâ
8
6
¨visible lightÆ
4
Bä
blue
0
0
200
red
400
600
800
wavelength in nanometers
1000
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1200
10. What are the wavelengths with the highest intensity for the bright source A and
the dim source B plotted on page 4? What is the corresponding brightest color
for each source?
11. Wien’s law says that the temperature T of a light source producing a continuous
spectrum is inversely proportional to the location of the peak of a light intensity
curve (the wavelength of light with highest intensity,
T = (2.89 × 106 K nm) /
λmax):
λmax
Estimate the temperature of the plotted on the bright setting (A) and on the dim
setting (B). Which is hotter? Show your work below:
12. Betelguese is a Red Giant Star found in the constellation Orion. Sirius, the
brightest star in the sky, is much hotter than Betelguese. How do you expect the
colors of these two stars to differ?
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Hydrogen Gas Lamp.
A gas lamp is filled with a diffuse gas. Electricity is used to excite the gas, adding
energy to the electrons. As the electrons return to their original energies, they emit
light.
1.
What color does the hydrogen lamp appear to be when you view it with your
eyes?
2.
What type of spectrum do you see when you look through the spectrometer at
the hydrogen gas lamp?
3.
Carefully make a sketch of the spectrum that you see through the spectrometer
on
scale
below.
Draw
verticalthat
pencil
line
at each
wavelength
where
3. the
Carefully
make
a sketch
of theaspectrum
you see
through
the spectrometer
on the
scale you
observe
linea vertical
in the pencil
hydrogen
spectrum.
Label
line with
color.
below. a
Draw
line at each
wavelength
whereeach
you observe
a line its
in the
hydrogen spectrum. Label each line with its color.
700
600
500
400
nm
Identify
the line corresponding
to the 1.9 eV
that we discussed
class. Remember
A bright4.line
is produced
when electrons
intransition
the hydrogen
atomsinmove
from a
that
particular high energy level to a particular low19energy level. In one of these transitions,
Energy in eV ⇥ 1.6 ⇥ 10
= h ⇥ frequency,
the electron loses 1.9 eV of energy. This energy is converted to light.
and
c = frequency ⇥ wavelength,
The energy of light is proportional to frequency ( E = h f ), and the frequency is
34 m2 kg/s is Planck’s constant and c = 2.998 ⇥ 108 m/s is the speed
where h = 6.626 ⇥
inversely proportional
to10wavelength
( f = c / λ ). By combining these formulas, and
9
of
light.
Recall
that
1
nm
= 10
Circle the lineof
in the
yourlight
drawing
above and show
converting units, one can show
thatm.wavelength
produced,
λ, is your
inversely
working 700
below.
600
500
400
nm
proportional to the energy, E:
λ = ( 1240 eV nm) / E
4.
Calculate the wavelength (in nm) that corresponds to 1.9 eV. Show your work
below, then circle the line in the drawing above.
700
600
500
400
nm
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Sources A and B.
Sources A and B are emission tubes set up near the front of class. One of these tubes
contains mercury vapor and the other contains helium gas. They are both hot gas
Sources
A so
and
B. both produce emission spectra. Make a careful sketch of the emission
sources,
they
spectraAviewed
the spectrograph
on the
the front
scalesofbelow.
Makeofsure
do contains
not
Sources
and B through
are emission
tubes set up near
class. One
theseyou
tubes
mix up sources
A the
andother
B, or contains
you willhelium
not be gas.
able They
to answer
thehot
questions
below.
mercury
vapor and
are both
gas sources,
so they both
3. Carefullyspectra.
make a sketch
of the
spectrum sketch
that youof
seethe
through
the spectrometer
on the scale
produce emission
Make
a careful
emission
spectra viewed
through the
below. Draw a vertical pencil line at each wavelength where you observe a line in the hydroSource
A
spectrograph
on
the
scales
below.
Make
sure
you
do
not
mix
up
sources
A
and
B,
or
you
will not
gen spectrum. Label each line with its color.
be able to answer the questions below.
Source A
700
600
500
400 nm
700
600
500
400 nm
4. Identify the line corresponding to the 1.9 eV transition that we discussed in class. Remember
Source B
3. that
Carefully make a sketch of the spectrum that you19see through the spectrometer on the scale
Energy
in at
eVeach
⇥ 1.6wavelength
⇥ 10
= where
h ⇥ frequency,
below. Draw a vertical pencil
line
you observe a line in the hydrogen
spectrum.
Label
each
line
with
its
color.
Source
B
and
c = frequency ⇥ wavelength,
where h = 6.626 ⇥ 10 34 m2 kg/s is Planck’s constant and c = 2.998 ⇥ 108 m/s is the speed
of light. Recall that 1 nm = 10 9 m. Circle the line in your drawing above and show your
working below.
700
700
700
700
600
600
600
600
500
500
500
500
400 nm
400 nm
400 nm
400 nm
The tables4.below
shows
list of sometoofthethe
spectral
lines
emitted
by the
elements helium
the
lineacorresponding
1.9 visible
eV spectral
transition
that
we
discussed
inby
class.
The tablesIdentify
below
show
some of the visible
lines
emitted
theRemember
elements
and
mercury:
that
helium and mercury:
Energy in eV ⇥ 1.6 ⇥ 10
= h ⇥ frequency,
400 nm
Element
447
Mercury
502
546 and c = 2.998 ⇥ Mercury
where h =
6.626 ⇥ 10 34 m2 kg/sHelium
is Planck’s constant
108 m/s is the speed
9
of light. Recall
m. Circle the579
line in your drawing above
and show your
588 that 1 nm = 10 Helium
Mercury
working 700
below.
600
500
400
nm
Helium
668
706
Helium
and
700
Wavelength (nm)600
Element
19
700
700
500 (nm)
Wavelength
c=
frequency ⇥436
wavelength,
Helium
600
600
500
500
400
400
nm
nm
Using
yourdrawings
drawings of
of the
the spectra
the
tables
above,
determine
whichwhich
element
is in each
1.1.Using
your
spectraand
and
the
tables
above,
determine
element
is
emission
tube
A
and
B.
Circle
the
correct
answers
below.
in each emission tube A and B. Circle the correct answers below.
700
600A:
Source
Source A:
700
600
Source
700
600
700
600B:
Source B:
Mercury500
Helium
500
500
500
Mercury
Mercury Helium
3
Mercury
400 nm
Helium nm
400
400
nm
400 nm
Helium
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700
600
500
400
nm
Fluorescent Lamp.
Observe the white fluorescent lights overhead through your spectrometer. Notice that
although the spectrum has some continuous regions, there are bright lines in the
spectrum. A fluorescent tube is a gas-discharge lamp that uses electricity to excite a
gas inside the lamp, which emits visible light as well as high-energy ultraviolet light.
The ultraviolet light causes a phosphor coating on the inside of the lamp to fluoresce,
producing additional visible light. In this lab, we want to determine which gas is inside
the lamp making the phosphor coating fluoresce.
1. Carefully make a sketch of the spectrum that you see through the spectrometer on
the scale
below.make
Draw
a vertical
pencil that
lineyou
at see
each
wavelength
where
observe
3. Carefully
a sketch
of the spectrum
through
the spectrometer
on you
the scale
below. lines
Draw ain
vertical
pencil line at of
each
wavelength
where you
observe a line in the hydrothe brighter
the spectrum
the
fluorescent
lamp
gen spectrum. Label each line with its color.
700
600
500
400
nm
4. Identify the line corresponding to the 1.9 eV transition that we discussed in class. Remember
that
Energy in eV ⇥ 1.6 ⇥ 10 19 = h ⇥ frequency,
2. Once you have observed all the spectra in the lab (the white light, source A and B
and theand
hydrogen lamp), compare the florescent light spectrum to the spectra from
c = frequency
⇥ wavelength,
the previous exercises to determine
what gas
is inside the fluorescent lamp. Note
34 m2 kg/s is Planck’s constant and c = 2.998 ⇥ 108 m/s is the speed
where
h
=
6.626
⇥
10
that the phosphor coating will add additional wavelengths of light to the gas
9 m. Circle the line in your drawing above and show your
of light.
Recall
thatanswer
1 nm = 10below.
spectrum.
Circle
your
700
working below.
600
Hydrogen
500
400
nm
Mercury
Helium
Explain your reasoning:
700
600
500
400
nm
700
600
500
400
nm
8 of 9
3
Star spectrum: (requires observation of light bulbs)
Below is a spectrum that was measured from a particular star. The intensity of light at
different wavelengths is plotted for a range of wavelengths including visible light.
Several dips in the intensity are labelled. Note that the wavelength increases from left
to right in this plot, while the spectrometer showed you wavelength decreasing from
left to right.
[nm]
[
5.
What type of spectrum is plotted in the figure?
6.
At approximately what wavelength is the star emitting the most light? What
color is this?
7.
How do you think the temperature of this star compares to the light bulbs you
observed earlier? Why?
8.
One dip in intensity is labelled “Na”, for sodium: this star has sodium gas in its
atmosphere. If you made a sodium vapor lamp, it would emit light at the same
wavelength as this dip. What wavelength is this, and what color would a sodium
vapor lamp appear?
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