The Question:
What is electromagnetic radiation?
Background Information.
Have students examine photos of the sun via X-Rays, UV, Visible light: http://www.exploratorium.edu/spectra_from_space/sun.htmlMost of the electromagnetic spectrum is invisible to our eyes, but these simple experiments elegantly show the existence of longer-wavelength infrared radiation, and shorter-wavelength ultraviolet light. While creating a spectrum is enjoyable, an exciting part is exploring the spectrum that is invisible.
Have you ever had X-rays taken? What did they look like? What could you see? Did you see the X-rays coming out of the machine? How did you know they were there? X-rays are part of the spectrum that are invisible. Today, you will prove the existence of invisible radiation. Remember, radiation means energy. These invisible rays are beyond both the red and violet ends of the visible spectrum and were discovered in the 1800's, when Johann Ritter proved the existence of ultraviolet rays and William Herschel did the same with infrared radiation.
You will be duplicating the same experiment as Herschel, when he demonstrated the existence of infrared radiation. To demonstrate the existence of ultraviolet radiation, Ritter performed an experiment using paper treated with silver chloride, which decomposes in the presence of light. He found that the silver chloride, deteriorated even more rapidly when exposed to the previously unknown radiation beyond the violet end of the spectrum, which the human eye cannot detect.
Procedure for Herschel's Experiment/Infrared radiation.
1. Allow 3 thermometers to register the temperature of the air where the experiment will be done - about 5 minutes. Take careful note of the temperatures.2. Create a spectrum using a prism or diffraction grating and sunlight as the light source. To do this requires the prism to be placed at an angle whereby sunlight enters the prism and then upon exiting is separated into spectra. The prism should be resting on a stable object so that the spectrum does not move.
3. Place thermometers at several points in the spectrum: one in the violet range, one in the center, and one just barely beyond the red end. Leave the thermometers in the spectrum for at least 5 minutes. Moving the thermometers carefully as the sunlight moves the spectrum. Temperature changes may be very slight, so observe carefully.
4. Make a data table and record your data.
Procedure for Ritter's Experiment/Ultraviolet radiation.
1.Create a spectrum using a prism or diffraction grating and sunlight as the light source. Since glass blocks most ultraviolet radiation, you must use an open window. The prism should be resting on a stable object so that the spectrum does not move.2. Working quickly to prevent exposure of the paper to too much light, cut a piece of blueprint paper about four times larger than the spectrum. Place blueprint paper, which behaves the same way that Ritter's silver chloride paper did, underneath the spectrum. Quickly outline the area covered by the spectrum with a felt-tip pen. Label the violet end.
NOTE: Depending on the sensitivity of the paper, different exposure times will be needed. Most exposure times will be fairly brief, however: about 15 to 20 seconds.
3. Put just enough ammonia in the pan to cover the bottom to a depth of about 0.5 in. (1cm). In front of an open window or beneath a vent fan, hold the paper over the pan of ammonia so that the fumes will process the paper. Notice the changes in the area outlined and the area just beyond the violet end. You may have noticed that this area began to change even before processing with the ammonia.
4. Record your observations and answer questions in the attached assessment.
Assessment.
Students complete the questions below.Questions for Infrared 1. What were the final readings on each of the 3 thermometers?
2. Why would there be an increase in temperature beyond the red end of the spectrum?
3. What does this tell us about what exists beyond the visible red?
Questions for Ultraviolet Radiation 1. What happened to the part of the paper lying where you can see violet?
2. What happened to the part of the paper lying just beyond that violet section?
3. What does this demonstrate about the area beyond the violet end of the spectrum?