Blackbody Spectrum PhET
Astronomy 2012
Name
Period
Date
Go to the PhET icon on the desktop and open the program. This program uses Internet Explorer
independent of the internet. So even though it looks like you are on-line, you are not.
INTRO NOTES:
1. Angstroms (Å), nanometers (nm) and micrometers (or microns, μm) are all units for very short
distances like the wavelength of light.
2. One micron is one
of a meter and one angstrom is
of a meter while one nanometer is one
of a meter.
3. Visible light is in the thousands of angstroms: 1000 Å =
you can think of it a 1 μm =
nm.
nm =
4. Looking at the graph, the bottom axis is measured in
part of the spectrum is found between what wavelengths?
μm or
and the visible
&
5. Temperature is measured in units of K (kelvin) that is a measure of the “absolute” temperature
of an object. Many application of science require that there are no negative numbers on the
temperature scale so the kelvin (or absolute) scale is used. The kelvin scale is just the Celsius
scale shifted by 273 degrees, so water that boils at 100 °C boils at 373 K on the kelvin scale.
Notice the number of Celsius degrees is 273 less than the kelvin value. What is initial
temperature?
K=
°C.
6. The “starburst” at the top of the screen represents the color that you would see if you saw a
blackbody heat source that had the temperature shown. They have simplified the spectrum into
three colors, blue, green and red. Which of these colors appears to be the brightest?
7. Look at the curved red line on the graph. This line represents the intensity of all of the “light”
given off by an object at the temperature shown. Remember, to an astronomer “light” is all
wavelengths not just visible, but also invisible. Is the red line at the same level for each of the
three colors (blue, green & red) listed next to the starburst?
8. Where is infrared located on the graph, left or right of the spectrum?
Would an
infrared sensor be able to detect this source? Explain
9. Where is ultraviolet located on the graph?
Would an ultraviolet sensor be able to
detect this source? Explain
10. The source in this simulation is emitting EM radiation “light” at a wide range of wavelengths.
What is the wavelength with the greatest intensity?
μm =
nm.
11. Wien’s law relates the peak wavelength of a blackbody source to its temperature in the
2.90  10 6 nm  K 2,900,000 nm  K

equation T 
. Use the wavelength in nm from #10 to
 peak ,nm
 peak ,nm
calculate the temperature of the blackbody in the simulation. How does it compare to the
value recorded in #5 above?
Now it is time to play with the simulation. Lower the temperature of the source.
12. Describe the change in the color of the source.
13. Describe the change in the shape of the graph.
14. How does the peak wavelength change?
15. What is the peak wavelength?
nm. Use Wien’s law to estimate the temperature of
the source. Show your calculation.
How does the calculated temperature compare to the simulation?
Now raise the temperature of the source.
16. Describe the change in the color of the source.
17. Describe the change in the shape of the graph.
18. How does the peak wavelength change?
19. What is the peak wavelength?
nm. Use Wien’s law to estimate the temperature of
the source. Show your calculation.
How does the calculated temperature compare to the simulation?
20. At what temperature would an oven heating element just begin to glow red?
K
21. What does this simulation assume is the average temperature of the earth’s surface?
What would this be in Celsius?
K
°C
22. We see the earth by reflected light, but why don’t we see the earth glowing except at a few
spots where there is an active volcano. Which of our senses would let us detect the EM
radiation being given off by the earth?
23. According to this simulation, what is the color of an operating incandescent light bulb?
24. Which spectral primary color is needed the least to produce the light bulb color?
25. Is an incandescent light bulb primarily a light source or a heat source? Explain
.
Wien’s Law Practice
http://astro.unl.edu/naap/hr/hr_background1.html
UV
V B G Y OR
IR
Star 1
Wien’s Law
𝑇=
2.90×106 𝐾∙𝑛𝑚
𝑝𝑒𝑎𝑘
Star 2
Star 3
Star 4
Wavelength
1. Use Wien’s Law to determine the temperature of each “star.” SHOW ONE EXAMPLE
CALCULATION. These curves are for ideal blackbody radiators (ideal stars) and NOT
real stars. We will discuss this later. Fill in the table.
STAR
Estimated Peak
Wavelength (nm)
Temperature
(K)
Predicted Color
of Star
1
2
499.6
3
4
2. Which star has the highest temperature according to Wien’s Law?
3. The hottest star would appear to be what color?
4. By looking at the graph, predict the color of each star and record it in the table above.