Blackbody Radiation

ASTR 181 Lab Write Up: Blackbody Radiation Spring 2023 Names: Corey B , Brian H Group # 3 When answering questions, please respond as concisely as you can. When answers must be calculated, show all work and answer all questions. Use scientific notation in E-notation format. Show units when they are not expressed in the step. Objective In this lab exercise, you will use a simulator to investigate some generic properties of blackbodies radiation and Wein's Displacement Law. Background Everything emits Electromagnetic (EM) radiation. The Sun does, operating incandescent lights do, we do, buildings do (stand near a brick building's west wall just after sunset; you can feel the heat radiating!). Anything at a higher temperature than its surroundings gives off some kind of EM radiation. The amount of power liberated in the form of EM radiation depends on the object's temperature (which we can measure in degrees or Kelvins) and its size (surface area, which we can measure in square meters (m 2 )). As you will find out, the wavelength of the EM radiation is related to temperature. The Range of Visible Light Access the PhET Blackbody simulator . You will be presented with the Planck curve of a blackbody at a temperature of 5800 K (approximately the same temperature as the surface of the Sun). The plot is of Spectral Power Density (essentially, the "intensity" of the radiation measured in megawatts per square-meter per micron) vs. wavelength (measured in microns or millionths of a meter). Consider the visible part of the spectrum. What is its approximate wavelength range? To make our measurement easier, check the Graph Values checkbox in the upper right, then drag the slider that appears on the curve toward the violet and red edges of the visible spectrum. As you do so, the intensity and wavelength values of the slider's position will update. 1. Find the approximate range of wavelengths of the visible spectrum, and estimate your numbers to the nearest hundredth of a micron (don't forget to include leading zeros before your decimal point if necessary). Violet starts at 0.397 Red starts at 0.638 Converting Between Units One micron is one-millionth of a meter or 10 -6 meter. To convert microns to meters, multiply the wavelengths from step 1 by 10 -6 meters. For example, we can convert 0.42 µm into meters as: Express your answer in scientific "E" notation. We can express your answer in E-notation format by entering 4.2E-7. 2. Convert your range of visible wavelengths in step 1 into meters. Use E- notation format.

Violet 4E-7 = (0.40um)x(10^-6m) Red 6.4E-7 = (0.64 um)x(10^-6m) In Physics, most wavelength measurements are in nanometers (nm) or 10 -9 meters. 3. Convert your range of visible wavelengths into nanometers. Violet 4E-9 nanometers (0.64 um)x(10^-9m) Red 6.4E-9 nanometers (0.64 um)x(10^-9m) The temperature of this blackbody is 5800 K, but how hot is that in more everyday terms? Fortunately, converting kelvins into degrees Celsius is pretty straightforward, as the difference is precisely 273.15. In other words: While converting into degrees Fahrenheit is 4. Convert 5800K to degrees Celsius and Fahrenheit to within a hundredth of a degree. Celsius: 5526.85 Fahrenheit: 9980.33 Colors of 5800 K Blackbody The starburst symbol at the top of the simulator represents the color of the blackbody as seen by your eyes. 5. What color is this "blackbody" as you see it? White or light gray Of the three primary colors (blue, green, red), notice that the blackbody curve peaks somewhere in the visible part of the spectrum. 6. Which color is this object producing the most? In other words, where does the 5800 K curve peak? Green is produced the most at 5800 K. 7. At what wavelength is visible light intensity the most and the least? Most 0.509 m m Least 0.772 m m Detecting EM Wavelengths Outside of the Visible Range 8. Would a camera that responds only to infrared light be able to detect this EM source? Explain why or why not. No, because if it only responds to infrared light it wouldn’t be able to detect visible light. 2

17. What's the difference between how light from Earth is produced versus light from volcanic lava? Earths light is produced because the sun shines on it and volcanic lava has light because its produces heat which in turn outputs light. Using the UNL NAAP Blackbody Simulator Go to the University of Nebraska Lincoln's Astronomy Applet Project (NAAP) web page by double-clicking on the NAAP icon on the lab computer. From the menu, click Blackbody Curves and UBV Filters . Open the Blackbody Curves and Filter Explorer . The temperature of the blackbody can be adjusted using the horizontal scrolling bar to the upper right of the blackbody graph. The "rainbow" represents the range of wavelengths viewable by humans. Underneath the temperature scroll, check the highlight area under curve and indicate peak wavelength boxes. Set the temperature to each of the values listed in table 1 and record the peak wavelength for each blackbody from the plot. 18. Write your peak wavelengths in nanometers into the table below. Temperature (K) Peak Wavelength (nm) 15,000 193.2 13,000 222.9 11,000 263.4 9,000 322.0 7,000 414.0 5,000 579.6 4

19. How does the peak wavelength of the blackbody vary with blackbody temperature? The wavelength get longer as the temperature decreases. Wein's Displacement Law Wein's Displacement Law states that the peak temperature of a blackbody is inversely proportional to its temperature: W here b is Wein's displacement constant, through careful experiments, this constant has been calculated with very high precision (2.897771955...×10 6 nm K). This was done by carefully measuring the actual peak wavelength of a physical object at a given temperature. With those independent measurements, the displacement constant can be calculated by simply multiplying the temperature by the measured peak wavelength: While making such precise measurements is not possible with our simulator, we can nevertheless determine the displacement constant using the peak wavelengths we determined in the previous exercise for each blackbody's temperature. For example, suppose you determine that a blackbody at 6000K has a peak wavelength of 483.0 nm. From this measurement, Wein's displacement constant is: Notice that the answer is computed to three significant figures. 20. Complete table below using the temperatures and the corresponding peak wavelength values from step 18 to determine b . Enter your result using E-notation (e.g., 2.897e+6) Note: you only need to show calculation for one of the temperatures. 15000x193.2=2,898,000 Temperature (K) b (nm k) 15,000 2.898x10^6 13,000 2.898x10^6 5