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Blackbody Radiation Astronomy 110
Background
Here you will investigate the properties of light from a source called a blackbody. A black body is defined as any object that absorbs all light that hits it. Blackbody radiation refers to the spectrum of light that a blackbody emits. All solid bodies and dense liquids and gases emit a blackbody spectrum, so that includes stars, planets, dense gases in space, people, light bulbs, and so on.
The electromagnetic energy absorbed by a blackbody is radiated back out at different wavelengths. Using an on-line experiment, we will investigate the light emitted. By analyzing the light produced by a blackbody source we can determine a fundamental property of the source. In astronomy this is very powerful because we can never travel to the objects we can try to understand. We only can receive the light they produce.
Procedure
1. Open the following link: https://phet.colorado.edu/en/simulation/blackbody-spectrum 2. Click the play icon to open the simulation.
3. A window will open similar to the one below. Notice there are two zoom button on the vertical axis intensity, and the same to buttons towards the right of the horizontal axis, wavelength:
 

What happens to the scale and to the plot when you click on the
What happens to the scale and to the plot when you click on the
What happens to the scale and to the plot when you click on the
What happens to the scale and to the plot when you click on the Does the spectrum represented by the red curve change in value
button on the intensity axis?
button on the intensity axis?
button on the wavelength axis?
button on the wavelength axis? by any of these actions?
 
Slide the temperature button around in both up and down directions:
Lower the temperature of the source. Describe the change in color of the source.
Describe the change in shape of the graph.
How does the wavelength of the maximum intensity change?
Raise the temperature of the source. Describe the change in color of the source.
Describe the change in shape of the graph.
How does the wavelength of the maximum intensity change?
Notice how the red blackbody curve sometimes goes through the top of the window. Notice how the curve sometimes is too low to be examined. When this happens, play with the zoom buttons to readjust the scale so that you have the curve plotted within the boundaries of the axes.
Fill out the following table using the tool:
Object
Temperature (K or Kelvin)
Wavelength at which the spectrum peaks, in other words, the color produced most by the object (m or micrometers)

Earth

Light bulb

Sun

Sirius A

Which object produces the most violet light?
Does this object have a high or low temperature compared to the others? Which object produces no violet light?
Does this object have a high or low temperature compared to the others?
Using your calculator or multiplying by hand, find the product of the temperature and the wavelength for each object and write your answer in the blank below.
Earth:
Light bulb:
Sun: Sirius A:
What do you observe about all of these answers?
If you observed a star with a device that could tell you the wavelength of light that the star produced most and it was 0.7 m, could you predict what the surface temperature of that star would be? Using the trend found in the products above try to predict what it would be.
Temperature Prediction:
Take the average value of all the products above (add all four numbers, then divide by four) and write
it here:
Use T to represent temperature. Use to represent wavelength.

Write the product of temperature and wavelength using the representative symbols:

Rewrite the product above and set it equal to the average value you found:
Compare your equation to Wien’s Law: T = 2900 m K


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