Solar Radiation (this spreadsheet may be helpful)
Use an visible wavelength range spectrometer covering from 350 nm to 1000 nm (more or less).
Be sure the software makes the wavelength axis visible with large fonts.
Discuss a little bit about how diffraction gratings work, and dispersion of light, maybe using a CD or DVD, or simple diffraction grating to make the point.
It may be useful to
demonstrate the spectrometer made from plumbing parts, with a slit on the end, diffraction grating or CD, and eye as detector.
Look at the spectra of 405 nm, 650 nm, and 532 nm laser pointers. 532 nm laser pointers also often have 808 nm and 1064 nm lines from the way they are made.
Talk briefly about the lasers in general (explain what lasing is all about), laser diodes, and the optics of the 532 nm laser pointer.
Measure fluorescence of paper, or a construction piece of clothing, etc, using the 405 nm laser diode as the source.
Look at the spectrum of light from a cell phone light source. Show how it is enhanced in the blue, and that the broad part of the spectrum extends to about 700 nm.
Discuss how the light is made from the cell phone light source; LED to drive a fluorescing material, and a decent use of electrical power since the light is covering the visible
spectrum, but not the near IR or UV.
Then show the spectrum of a flashlight that uses an incandescent light bulb. Notice how a large amount of the radiation is in the near IR, not useful for lighting for the human eye
as a detector. Notice that the light warms bare skin because the near IR is absorbed. Talk about the blackbody nature of this radiation source, and the relatively lower efficiency for visible light production compared with the LED source. Then proceed with the questions below, using the spreadsheet mentioned above.
1. What wavelength range comprises solar radiation at the Earth's surface?
2. How is solar radiation affected by the Earth's atmosphere and surface?
3. What is the equivalent black body temperature of the Sun with regard to the solar spectrum we see at the top of the Earth's atmosphere?
4. How much solar radiation (irradiance) is present at the top of the Earth's atmosphere in watts per square meter units?
5. How many one square meter,
20% efficient solar panels would be needed at the top of the Earth's to run a 2000 watt hair dryer?
6. What are the primary atomic and molecular properties are involved with absorption of solar radiation at wavelengths from the UV to the near IR?
7. What is the peak wavelength of solar radiation?
Infrared Radiation (review the link in the title to obtain the demonstrations made using the IR camera).
8. We did a 'high five' in class by pointing our hands towards each other and letting the infrared photons fly. Assuming human skin is a perfect blackbody emitter and absorber, and that the hand is at normal body temperature (expressed in Kelvin units), use the Stefan Boltzmann law to calculate how much radiation in watts per square meter is emitted by your hand. What fraction is this compared with the solar radiation at the top of the atmosphere?
9. Use Wien's displacement law to calculate the peak wavelength of infrared emission by your hand, using the same temperature from question 8, and the sun's equivalent blackbody temperature from question 3.
10. Suppose that one hand was dipped in water and then you waved dry air over your hand to cause it to evaporate. What does the infrared camera show for your hand show for your wet hand temperature compared with your dry hand temperature, and why?
11. When looking at a person with an infrared camera, why is the face warm while the hair, nose, glasses, and clothes look cool by comparison?
12. In the experiment with a small square of aluminum foil put on a student's forehead, it was observed that the aluminum foil looked
dramatically cool, even though the aluminum foil is at the same temperature as the forehead. From this observation we can learn about how aluminum interacts with infrared radiation: describe this.
We were able to write a message on the wall just by vigorously rubbing it in the pattern of letters, and observing it with the infrared camera. How does this work? Could we make a cool message by putting a thin film of water on the wall and evaporating it? (try this).
We did a demonstration with the dinner plate that was transparent at visible wavelengths, and argued that it must be a strong absorber because we can't see the infrared radiation emitted by a hand if the plate is between the hand and the camera. Describe how the plate is like the Earth's atmosphere in its effect on visible and infrared radiation, and the 'greenhouse' effect. Which gases in the atmosphere are infrared active as 'greenhouse' gases?
15. We did a demonstration of visualizing atmospheric convection by having the camera visualize the hot air, from use of a propane torch below the camera field of view, rise into the field of view of the camera. We also did a demonstrate of thermal conduction by heating the glass rod with the propane torch. Which process is more effective at transferring heat in the atmosphere above the Earth's surface? By the way, why does the hot air rise? And why is the hot air visible -- what are the gases produced by combustion of the propane, and are they infrared active gases that can absorb and emit infrared radiation?
16. As time permits, we will also fill a transparent container with water and use the infrared camera to view the water. Is water a strong absorber and emitter of infrared radiation? How can you figure this out? Can you observe thermally induced convection in the water using the camera? Would you expect ice to behave the same as water in its interaction with infrared radiation?
This laboratory (pdf version; MSword version) further explores the role of infrared radiation in climate, and the impacts of infrared active species such as water vapor, carbon dioxide, and clouds. The summary graph is here, based on the model approach described in this lab exercise. Click on the image for a larger version.
Here is a classic paper that looks at this issue is a more comprehensive manner.
Here is a counter point to the classic paper.
Here is an update of the classic paper.