Astronomy 110 PRINT Name   __________________________
Fall 2005   Section 006  

Homework 6 : Measuring Temperature

(Due Thursday, Oct 20, 2005)

The spectrum of sunlight, the "rainbow", is related to the ideal "black body" spectrum, three of which are shown below. For a "black body" (an ideal emitter), once you know the temperature, the amount of light emitted from each square centimeter of its surface is known exactly. The three plots below are the intensity of light as a function of wavelength in Angstroms for three black bodies at different temperatures. The range of visible wavelenths of light is also shown. Answer the questions on the back.

  1. Mark on the diagram which is the blue end and which is the red end of the visible wavelengths.

  2. Measure the wavelength where each of the spectra, A, B and C, is at a maximum and enter it in the table below. Obviously you won't be able to measure very accurately, particularly with curve C, but do the best you can. It would be best to use a metric ruler with fine divisions. Also, try to make a realistic estimate of how well you can actually measure these wavelengths, and have that reflect in the numbers you enter in the table. For example, could you really measure a wavelength of 11,001 Angstroms, or can you only say that it is nearly 11,000 Angstroms (as far as you can measure it). If not, enter 11,000 Angstroms, not 11,001 Angstroms.

             A              B              C      
    Peak Wavelength in Angstroms                        
    Temperature in Kelvin                        

  3. Now you are in a position to measure the temperatures of bodies A, B and C, which you do by using Wien's Law which says that the wavelength of peak intensity (that you just measured), w (in Angstroms) is related to the temperature, T (in Kelvin) by

    w = 3 x 107 / T

    Calculate the temperatures of bodies A, B, and C and enter them in the table above.

  4. Which curve is closest to representing the light from the Sun?    A    B    C

  5. Consider the visible light portion of curves A and C. Which of these curves represents a hot body that is giving off light that looks blue to your eye?    A    C

    Reason :