AST2100L Labs Page

THE NATURE OF LIGHT
(100 points)

PURPOSE: to understand how light originates and how we use it to study celestial objects

MATERIALS: calculator, spectroscope, spectra graph paper (2 sheets)

INSTRUCTIONS: complete your prelab, print out these pages, and complete the activities below

SECTION 2 ACTIVITIES :

2.1. (1 pt) Put these regions of the electromagnetic spectrum in order starting with the shortest wavelength :
visible, x-ray, radio, infrared, gamma ray, ultraviolet.

2.2. (1 pt) Which wavelengths are more energetic - radio waves or gamma rays?

2.3. (3 pts) If we measure an energy of E = 4.675 x 10^-19 kg m² / s², then what was the observing wavelength l used?

2.4. (1 pt) What region of the EM spectrum does the result in #2.3 correspond to?

2.5. (8 pts) Match each of the following wavelengths with their corresponding region of the EM spectrum.
Use Figure 4 in the lab text to help you.

a) 6.563 x 10^-7 m __________

   i) UV (ultraviolet)

b) 2.4 x 10^-13 m    __________    ii) IR (infrared)

c) 0.21 m    __________    iii) hard x-ray

d) 5 x 10^-5 m    __________    iv) microwave

e) 1 x 10^-9 m    __________    v) gamma ray

f) 3.66 x 10^-8 m    __________    vi) visible

g) 1 x 10^-2 m    __________    vii) radio

h) 9.45 x 10^-11 m    __________    viii) soft x-ray

SECTION 3 ACTIVITIES :

3.1. (3 pts) Use Wein's law to calculate the approximate blackbody temperature which corresponds to a
peak wavelength of l_PEAK = 4.25 x 10^-7 m.

3.2. (3 pts) If an object has a temperature of T = 30,000 K, calculate the best wavelength to observe the object at.

3.3. (3 pts) If object A is found to put out a flux of F = 1.453 x 10⁶ W/m², what is its approx. temperature?

3.4. (3 pts) Object B has a flux F = 4.24 x 10¹² W/m². How many times hotter is object B than object A?

3.5. (1 pt) Wein's law yields __________ temperature, but the Stefan-Boltzmann law yields __________ temperature.

a) effective; surface
b) blackbody; effective
c) effective; blackbody
d) interior; effective

3.6. (2 pts) The Sun has a temperature of T ~ 5800 K and a peak wavelength of l_PEAK = 5 x 10^-7 m.
Is the Sun a blackbody? Why or why not?

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**EXPERIMENT**
Let's play with blackbody spectra using the "Blackbody Radiation and Stellar Luminosity" applet, © McGraw-Hill, Inc.

Before starting, open the applet and familiarize yourself with it. Along the lefthand side of the window,
the framed area contains review topics if you need. On the applet itself, take note of the temperature
selection bar at the bottom of the window and the stellar data box in the upper righthand corner which
shows the type of star needed to produce the spectrum you create. The default is for T = 5476 K and
a star like our Sun, M = 1 M_SUN and R = 1 R_SUN. Move the temperature selection bar back and forth,
watching how the graph and stellar data changes until you understand how the applet works. Note that
there are only specific temperature steps available (e.g, 5476 and 5625, but not any values in between).
So when asked to create a curve for a specific temperature, just select the temperature that is closest to
what you need. Pressing the 'RESET' button will return the applet to the default (solar) blackbody curve.
Pressing 'NEW GRAPH' freezes the current graph and allows you to create others and compare.

3.7. (2 pts) Press 'RESET.' The solar blackbody here has a temperature of T_SUN = 5476 K.

a) Which region of the electromagnetic spectrum does the Sun peak in?

b) Estimate the peak wavelength (in nm) for the Sun from this graph.

3.8. (3 pts) The giant star Rigel (in the constellation Orion) has a temperature of T ~ 15,000 K.
Another star in Orion called Betelguese has T ~ 2,200 K.

a) What region of the electromagnetic spectrum is best to observe Rigel?

b) What region of the electromagnetic specrum is best to observe Betelguese?

c) Describe the difference in the colors of these two stars.

3.9. Suppose there are two stars with T = 25,000 K. Create the blackbody curve for both.

a) (1 pt) Now suppose star 1 is a giant (R = 2.5 x 10¹ R_SUN) and star 2 is a dwarf (R = 1.0 x 10^-1 R_SUN).
Below, record the luminosities they both must have in order to remain at T = 25,000 K :

STAR 1 : ____________________ L_SUN STAR 2 : ___________________ L_SUN

b) (2 pts) How many times brighter is the giant (star 1) than the dwarf (star 2)?

3.10. (1 pt) Based on your experiments with this applet, what type of relationship (direct, indirect, exponential, etc.) exists between stellar radius, temperature, and luminosity?

3.11. (1 pt) Hotter objects are more _____ in color and cooler objects are more _____ in color.

a) red; blue
b) blue; white
c) green; blue
d) blue; red

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**EXPERIMENT**
Now we will let's look at another "Blackbody Radiation" applet, © Univ. of Oregon JavaLabs.

Before starting, open the page, and scroll down until you see the applet entitled "Blackbody Radiation
and Stellar Temperature". Familiarize yourself with it. The spectrum graph covers a wavelength range
of Δλ of 2000 to 12,000 Å (or 200 - 1200 nm) which spans across the far-ultraviolet, visible, and near
-infrared regions of the electromagnetic spectrum. On either side of the graph there are temperature bars
which can be slid up and down with the mouse to create the corresponding blackbody curve. The default
temperature is T = 100K. Note also that if you place your mouse cursor anywhere on the graph and click
the right mouse button, coordinates will appear to show you the exact temperature and intensity at a given
point. Finally, notice along the bottom of the applet there are three boxes which correspond to three filters:
red (R), blue (B), and visual (V) which is essentially green. If at any time you want to clear the applet, just
press the 'Refresh' button in your internet browser.

3.12. (2 pts) Set the red thermometer to T = 11,100 K and the black thermometer to T = 7,010 K. In
terms of peak wavelength and maximum intensity (i.e., shape and size of curve), explain what happens
to a blackbody as you increase temperature.

3.13. Press the browser's 'Refresh' button to clear the applet. Activate the color filters by clicking on the boxes labeled 'Red', 'Blue', and 'Visual Band.'

a) (3 pts) Create blackbody curves for the six temperatures listed in the table below. and fill in the appropriate data. Be sure to press 'Refresh' before each new curve you create and to move your cursor back over the graph each time you create a new curve.

TEMP	B/R	B/V	V/R
1,933
3,061
7,010
11,100
18,575
27,038

b) (1 pt) How do the color values change as temperature is increased?

3.14. Based on your results above, answer the following questions:

a) (1 pt) What do you notice about B/R values as compared to B/V values for the lower temperatures versus the higher temperatures?

b) (3 pts) The difference in values of B/R as compared to B/V for you noted in part a) above occurs at around T ~ 8700 K. Do a calculation of peak wavelength. Use this to give a physical reason for the color difference.

3.15. (2 pts) Why is it easier for astronomers to make measurements of an object's color than to an object's peak blackbody wavelength?

SECTION 4 ACTIVITIES :

4.1. (3 pts) Look at the three spectra here. Based on Kirchoff's laws, determine whether they are
continuous, emission-line, or absorption-line spectra.

A.
B.
C.

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**EXPERIMENT**
Take a look at the periodic table to see what elemental spectra look like.

Open this laboratory spectra applet. First select either 'Absorption' or 'Emission' spectra by clicking the appropriate circle. Then to view spectra, just click on the element box you want. Start with Hydrogen
since it is the simplest atom. Examine numerous elements to gain an understanding of spectra.
Please focus on elements 1 - 89 only.

4.2. (1 pt) How are absorption line spectra and emission line spectra different visually?

4.3. (1 pt) Does the number of spectral lines increase or decrease across a row in the periodic table?

4.4. (1 pt) Does the number of spectral lines increase or decrease down a column in the periodic table?

4.5. (1 pt) In general, does the number of spectral lines increase or decrease with larger atomic number?

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**EXPERIMENT**
Now we will complete an experiment to actually see the spectra of specific elements in person.

Your TA will set up a power source and gas tubes. Do not touch the gas tubes as they will become very
hot! Obtain 2 pages of spectra graphs. Notice that each graph is labeled for one element. On the x-axis,
there is a scale for wavelength (color) which matches the scale inside your spectroscope.

4.6. (12 pts) With your spectroscope, look at each element the TA sets up. Draw the spectral line pattern
you see as accurately as possible on the provided pages of graphs. Each graph is worth 2 points. Be sure
to turn these pages in with the rest of these lab activities.

4.7. (2 pts) Which element showed the fewest spectral lines? Which element showed the most lines?

4.8. (1 pt) Afterwards, your TA will set up a “mystery gas”. Observe it, draw it (if you like), and
compare it to the previous six elements. Which element (or combination thereof) do you think the
"mystery gas" is?

4.9. (3 pts) Explain how spectral lines are generated and why a spectrum may have more lines than another.

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4.10. Here we show two spectra . The first is of a star called MWC 1080. The second is for the Sun.

a) (1 pt) Look at the MWC 1080 spectrum and estimate its peak wavelength (has units of Å).

b) (3 pts) Use Wein's law to calculate an approximate temperature for this star.

c) (1 pt) Compare this spectrum to that of our Sun and list 5 absorption lines (by wavelength) that are in both spectra.

d) (1 pt) Based on your answer to #4.6c, list the 4 chemical elements that are found in both MWC 1080
and our Sun, in order of most abundant to least abundant. (Hint: abundance is indicated by width and
strength of spectral line).

4.11. (3 pts) Why is spectroscopy important? List two types of information we can determine about a
celestial object by studying its spectrum.

* TURN IN THESE ACTIVITIES PAGES TO YOUR TA*

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