Black Body Radiation PhET

Directions

1. Click Here to run the PhET of the Black Body Spectrum.

2. When the Sim runs, check "Graph Values" and "Labels"

3. You are looking at the Black Body spectrum of the Sun when it starts up. Notice the shape of the curve, and how it has the long tail up into the longer wavelengths. This was the part of the curve the Planck had a hard time with.

4. Notice that the sim puts a white dot at the peak of the curve (unless you drag it), and labels this wavelength below the graph in yellow numbers. Since the x-axis is in micrometers, multiply the yellow number below the graph by 1000 to make it nanometers. What is the Peak (brightest) wavelength at the Sun's Temperature in nm? (5800 K) This wavelength is right in the middle of the visible Spectrum. This is not a coincidence.

5. Now change the temperature to 7500 K (from 5800 K) Notice that the intensity increases quite a bit. (Increases with T⁴) Click the "-" magnifying glass to zoom out. What is the new peak wavelength in nm? Did it increase or decrease? Is the light bluer or redder?

6. What temperature correspond to a peak wavelength of 700 nm? (The red end of the visible spectrum)

7. What temperature corresponds to a peak wavelength of 380 nm? (The blue end of the visible spectrum)