Lab 9 Notes and Lab Questions
For Lab 9, you won’t be doing anything out of the lab manual. (We’re skipping Lab 9, which requires a lot of in-person equipment). Instead, you’ll be using a simulation that shows the color and amount of light produced by objects at different temperatures (https://phet.colorado.edu/sims/html/blackbody-spectrum/latest/blackbody-spectrum_en.html ). The simulation can also be used to find the total energy produced by each object, if you know how big the object is.
You can do this lab on your own if you want, or you can choose to attend the online session on blackboard (Tu 9-11 or 11-1 on Apr 13th) for help. The youtube walkthrough will be posted on Wednesday here: https://www.youtube.com/playlist?list=PLqXGExy61NILKxSOpnD51PTf-8yYFxWvp
Part 1: Colors
All objects make their own light, even if that light isn’t visible to us. If an object is hotter, it produces higher energy kinds of light. In this part, you’ll learn how to use the simulation to determine what kinds of light is being made by objects at different temperatures. As usual, answer the number questions below as your lab data. Since there is no lab manual for this lab, just answer the questions and show your work in a document.
Follow the link to the simulation.
The graph in front of you shows the total amount of light produced by an object of a certain temperature. When you first load the simulation, it is set up to measure the light from the Sun.
1. Record the temperature of the Sun. Use google to convert the K temperature of the Sun into whichever temperature system you’re more familiar with (F or C). (Type “5800 K to F” or “to C”, depending)
The graph on the left shows the amount of light given off by the Sun (or any other object at 5800 K). The kinds of light are shown horizontally: click “labels” to see the names of the different kinds of light shown. The higher the curve is at a certain color or type of light, the more light is produced by the Sun of that color or type. We’ll use the specific colors when talking about the visual rainbow; use UV and IR to talk about ultraviolet or infrared light.
2. What color or type of light does the Sun make the most of?
Besides all of the colors of the rainbow, which we can see, the Sun also makes UV and IR light, which our eyes cannot see.
3. Does the Sun make more IR or UV? (Hint: compare how much of the Sun’s light curve is on the UV side vs the IR side.)
Our eyes don’t see the individual colors separately when we look at a bright object; instead, we see the combination of all of them together. The star shape at the top shows the rough color that we see from the Sun when all that light is combined.
4. What color do we see with our eyes from the Sun? Bright blue / red / green? Something else?
By dragging the triangle up and down on the thermometer, you can change the temperature of the object you’re looking at. This will sometimes make the graph too tall or short to see, so you’ll need to use the zoom buttons on the x- and y-axes to center the graph again.
First, we’ll look at Sirius A, a star that is hotter than the Sun. It is the brightest star in the sky. Turn the temperature up until it matches Sirius A.
5. Record the temperature of Sirius A. How hot is it in F or C (whichever you choose)?
The graph is should be off the top of the graph. Experiment with the plus / minus zoom buttons on the TOP of the graph: these affect the vertical zoom. (The buttons on the bottom adjust the horizontal zoom, which we’ll need later on) Adjust them until you can see the peak of the orange line again.
6. What colors and types of light does Sirius A make? What color or type of light does it make the most of? What color will we see when we look at Sirius A with our eyes?
Now adjust the temperature to match a light bulb.
7. Record the temperature of the light bulb. How hot is it in F or C?
Because the light bulb is much cooler than the Sun or Sirius A, you’ll need to adjust the zoom at the top again to see its light curve. Adjust until you can see the curve clearly.
8. What colors and types of light does the bulb make? What color or type of light does it make the most of? What color will we see when we look at the bulb with our eyes?
Now switch over to the temperature of the Earth. Because the earth is so much colder than the other objects, you’ll have to adjust both vertical AND horizontal zoom until the light curve is on the graph.
9. Record the temperature of the Earth. How hot is it in F or C?
10. The Earth only makes one kind of light. Which kind? Is it a kind we can see with our eyes?
Notice that the bright star that showed you the color of other objects is now gone: our eyes don’t see any light that is produced by the Earth.
11. About how hot would an object have to be to make more purple than any other color of light?
In the sky, there are no stars that look green to us.
12. Experiment with the temperature gauge. What are the possible star colors that we can see with our eyes?
Part 2: Calculating the luminosity
The total energy that is produced by the light coming from an object every second (its luminosity) depends on both its temperature and how big the object is. In this section, you’ll calculate the energy produced by each object every second.
Click on the intensity box. This expresses a number with the units “W/m2”. A Watt (“W”) is how much light energy the object gives off every second. This number is divided by “m2”; if you multiply the number you see by the area of the object in meters squared, you can get the total energy it releases in light every second.
On the graph, the area under the orange curve has been highlighted. The number in the intensity box represents the total energy produced by combining all of the kinds of light that each object makes.
To figure out how much energy the Earth makes, we need the surface area of the Earth. The Earth is a sphere, so the surface area of the Earth is given by:
Surface Area of a sphere = 4πR2
13. Look up the radius of the Earth in meters. Record that number.
14. Using a calculator or google, calculate the Surface area of the Earth. (This will be shown in detail in the lab video)
To get the total amount of light energy given off by the Earth each second, multiply your area number by the intensity number recorded on screen. Make sure to include the scientific notion part: the 10x power at the end of the number. If you’re not sure how to deal with this, follow the instructions in the lab video.
Luminosity = Surface area * Intensity
15. What is the luminosity of the Earth?
Notice that the Earth makes a lot of energy mostly because of its very large size.
Compare this to the energy produced by a light bulb. The actual piece of metal that glows to make the light in the bulb is very small: use 1.5 x 10-5 m2 for the area of the bulb.
16. How much light energy does the bulb produce each second? Is it more or less than the Earth? How close is it?
You can use the same technique you used with the Earth to figure out the total energy made by either of the two stars.
17. Pick either the Sun or Sirius A.
18. Look up the star’s radius in meters, and record it.
19. Calculate the area of the star, using the surface area of a sphere like for #13.
20. Record the intensity number of the star
21. Calculate the total energy made by the star every second, by multiplying the surface area by the intensity. Record your final answer as a number of Watts (W).
You can look up the luminosity of the star you chose; either the Sun or Sirius A. You’ll compare this number to the number you just calculated in the conclusion.
22. Look up the luminosity in Watts of the star you chose, and record it. .
Lab Questions for Lab 9
• Purpose (what is the point of the lab? Why did you do what you did?
• Ideas (explain the given idea; and one more that you pick)
• An observation you made in looking at the colors and types of light in Part 1.
• An observation you made while calculating total energy in Part 2.
• Conclusion (answer these questions in sentences. Use your lab data to support your answers)
-Compare the colors you found for the four objects in Part 1. How were the colors of light affected by the temperature of the object? How were the colors that we see with our eyes affected by the temperature of the object? Describe some of the details.
-How does the luminosity you calculated for your star compare to the number you looked up? From your perspective, did the equations you used work well? Explain.
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