Beyond the Rainbow

Introduction

Little was known about the electromagnetic spectrum at the beginning of the nineteenth century other than white light could be dispersed into the colors of the visible spectrum. In 1800, William Herschel (1738–1822) discovered infrared light and the following year Johann Wilhelm Ritter (1776–1810) verified the existence of ultraviolet light. By observing experiments similar to Hershel’s and Ritter’s, students can see for themselves there is more to light than meets the eye!

Concepts

• Electromagnetic spectrum
• Infrared light
• Ultraviolet light

Background

In 1800, William Herschel was investigating why different colored filters he used on his telescopes absorbed heat differently. Using a prism to produce a spectrum, Herschel placed a thermometer in one color of the spectrum and two other thermometers in the shade outside of the spectrum as controls. He moved the test thermometer to each color in the spectrum in turn, (violet, indigo, blue, green, yellow, orange and red), measuring and recording the respective temperatures. He noted that the temperature increased as the thermometer moved from the violet to the red end of the spectrum. As the angle of the sun changed over time, the spectrum shifted, causing the thermometer that had been in the red light to be in the shaded area just beyond the red. Herschel was very surprised when he looked at the thermometer and observed an even higher temperature.

Herschel concluded another form of invisible energy must exist beyond red visible light. Further investigation revealed this invisible radiation behaved just like light—it could be reflected, refracted, absorbed, and transmitted. Herschel called the radiation “calorific rays,” but today it is known as infrared (infra = below) light or infrared radiation.

As Herschel’s work became known, German chemist Johann Ritter hypothesized that invisible energy might also exist beyond the violet end of the spectrum. He used a chemical to test his theory. Ritter knew that paper treated with silver chloride would darken when exposed to light and that blue light caused a more vigorous reaction than red. As he exposed the chemically treated paper to the full spectrum of visible light, his hypothesis was confirmed. The silver chloride decomposed the fastest in an area beyond the violet end of the spectrum. He called this newly discovered radiation “chemical rays,” but later it was renamed ultraviolet (ultra = beyond) light.

Materials

Safety Precautions

The glass prism and thermometers may break or chip if dropped. Handle all glass items with care. Never look directly at the sun. Wear chemical resistant gloves when rinsing the sun print paper. Wash hands thoroughly with soap and water before leaving the laboratory. Follow all laboratory safety guidelines.

Disposal

All materials may be stored for future use. Used sun print paper may be thrown away in the regular trash.

Prelab Preparation

1. Blacken the four thermometer bulbs with black spray paint (flat paint is best) or a permanent black marker. If using spray paint, use masking tape to cover all but the bulbs of the thermometers to avoid any overspray.
2. Using a utility knife or heavy duty scissors, cut a notch at the top edge of the box at the narrow end. The notch should be slightly smaller in width and depth than the prism (see Figure 2). The prism should fit tightly in the notch and still be able to rotate.
   {12757_Preparation_Figure_2}
3. Place white paper flat inside the box covering the bottom and tape the paper in place.
4. Cut a flap in the lid of the box. The flap should line up with the prism notch in the box, and be ½" wider than the notch on each side. Extend the flap on the top of the box about 6 inches toward the middle. Slightly score the flap across the attached end (be careful to not cut all the way through), and again about 3 inches from the attached end (see Figure 3a).
   {12757_Preparation_Figure_3a}
   Do not cut off the front face of the flap (see Figure 3b)
   {12757_Preparation_Figure_3b}
5. Cut a sturdy piece of cardboard so the height and width is the same as the box. This will be used as a divider for Part 2 and should fit snugly in the box.
6. “Nest” three of the thermometers together, placing the Celsius sides over the Fahrenheit sides (see Figure 4).
   {12757_Preparation_Figure_4}
7. Cut or tear two pieces of tape long enough to go across all three thermometers with an inch of tape left over on either side.
8. Keeping the three thermometers close together, turn them over and place one strip of masking tape across the top and one strip across the bottom of the nested thermometers (see Figure 3). Make sure the degree markings line up on all three thermometers.
9. Cut one sheet of 9" x 12" black construction paper in half to make a 6" x 9" cover for the sun print paper. Note: Keep the sun print paper in the black plastic envelope provided until just prior to the Ritter experiment.

Procedure

Both experiments yield optimal results when conducted outdoors on a calm sunny day. The Hershel Experiment works best when the air temperature is below 80 °F.

Part 1. Hershel’s Experiment

1. Obtain the glass prism, a stopwatch or timer, the three “nested” thermometers, and another single thermometer. Hold the prism by the ends to avoid fingerprints. Place all four thermometers, the prism, stopwatch, and extra tape inside the prepared box and take the box outside to a sunny location. If doing both experiments, take the envelope of sun print paper, a black permanent marker, and the black construction paper outside also.
2. Remove the lid from the box and set it aside for Part 2.
3. Carefully place the prism in the notch at the end of the box (see Figure 2 in the Prelab Preparation).
4. Orient the box so the prism side faces the sun.
5. Rotate the prism until a clear, wide spectrum is visible inside the box. Note: The end of the box with the prism may need to be tipped up slightly so the spectrum will fall on the white paper. Books or a rock may be used to prop one end of the box if necessary.
6. Place all four thermometers in the shade. Wait 60 seconds and then record the temperature of each in Data Table A in the row labeled 0 minutes on the Beyond the Rainbow Worksheet. The single thermometer is for the shade temperature.
7. Place the single thermometer in a shaded portion inside the box and move the three nested thermometers to the spectrum. Place the nested thermometers in such a way that one thermometer is next to, but not in the red portion of the spectrum (see Figure 5). The other two thermometers should be in other colors of the spectrum (one in the violet/blue range and one in the orange/yellow range). Fold back the extra tape to secure the thermometers on the white paper in the spectrum. Start timing.
   {12757_Procedure_Figure_5}
8. After 1 minute, note and record the temperature of each thermometer in the data table.
9. Repeat step 8 until five temperature readings have been taken. Note: Depending on the time of day, as the position of the sun changes during the experiment (due to the rotation of the Earth), the spectrum may move slightly. Adjust the position of the box accordingly to keep one thermometer just beyond the red.

Part 2. Ritter’s Experiment

1. Remove the white paper from the bottom of the box and place a 9" x 12" piece of black paper on the bottom half of the box closer to the notched end.
2. Place the prism in the notch in the prepared box (Figure 2 in the Prelab Preparation section).
3. Orient the box so the prism faces the sun.
4. Place a divider in the center of the box and tape in place, making sure the bottom of the box between the prism and the divider is completely covered with black paper.
5. In a shaded area, remove one piece of sun print paper and quickly cover the blue side with a 6" x 9" piece of black paper.
6. Being careful to limit exposure to the sun as much as possible, tape the sun print paper to the top center of the divider and then cover the sun print paper with the black paper. Secure the black paper to the divider with a paper clip. Position the paper clip more horizontally than vertically so it will not interfere with the spectrum.
7. Rotate the prism until a clear, wide spectrum is visible on the black paper. Do not tip the box. Note: Depending on the angle of the sun, the spectrum may appear near the top of the paper. Orient the prism so at least 3 centimeters of the black paper are above the visible spectrum.
8. Place the prepared lid on the box, positioning the flap as shown in Figure 6 with the front section raised and the back section flat. As the sunlight goes through the prism, the spectrum should still be visible on the black paper. The rest of the black paper should be in shadow with no exposure to white sunlight.
   {12757_Procedure_Figure_6}
9. Being careful to not move the prism, lift the flap slightly and remove the black paper cover from the sun print paper by carefully pulling it out of the paper clip. The spectrum should now be visible on the sun print paper.
10. Without removing the lid, use a black permanent marker to carefully outline the visible spectrum. Draw an “R” at the red end of the spectrum and a “V” at the violet end (see Figure 7) for a view of the setup inside the box). Note: This step may be a little tricky. It may be necessary to lift the flap more in order to reach in with the marker. Just keep the white sunlight exposure time on the paper to a minimum.
    {12757_Procedure_Figure_7}
11. Reposition the flap as in step 8. The lower edge of the front face should rest on the top of the prism. The only light exposure on the sun print paper should be the spectrum.
12. Allow the sun print paper to be exposed to the full spectrum for 2 minutes. Continuously view the spectrum through the flap of the box to make sure the marker outline is still around the spectrum. If necessary, adjust the position of the box.
13. After 2 minutes, remove the prism, close the flap of the box, and take all the equipment back to the laboratory.
14. Once inside, remove the sun print paper from the box.
15. Wearing gloves, rinse the sun print paper under tap water for 1 minute.
16. Lay the sun print paper flat on paper towels to dry, exposed side up. Carefully blot excess water from the top of the sun print paper with a paper towel.
17. Observe the sun print paper. A blue area will develop where the paper was exposed to ultraviolet light. This will darken with time (check the paper again the next day). The area of the spectrum at the red end (marked with an “R”) should show little, if any, exposure. The dark area should extend beyond the violet end of the outline.

Student Worksheet PDF

12757_Student1.pdf

Teacher Tips

• This kit contains enough materials to perform the demonstration 15 times: one glass prism, four thermometers, 15 sheets of sun print paper and two sheets of black construction paper.
• A standard photocopier paper box works well for this demonstration. One large piece of white construction paper or two pieces of copy paper will cover the bottom of a box this size.
• The infrared activity is best conducted on a sunny day without much wind, with the shade temperature below 80 °F (27 °C). A greater difference in temperature among the thermometers will result if the shade temperature is even cooler.
• Since energy across the electromagnetic spectrum increases from red to violet, students may be confused to find increasing temperatures from violet to beyond red (an increase in wavelength). Remind students that heat is just one form of energy. For a more advanced explanation, see “Reconciling the Herschel Experiment” at the website home.znet.com/schester/calculations/ herschel/index.html (Accessed June 2008).
• In our experience, conducting the demonstrations indoors with sunlight through a window or with light from an overhead projector did not provide satisfactory results.
• A glass prism must be used as acrylic will block infrared rays.
• The Ritter experiment works best if the rays from the prism are as direct as possible. If the box is tipped, the spectrum will spread out and some scattered white light may appear beyond the spectrum.
• To keep the sun print paper from curling while drying, place the wet paper on cardboard and pin the corners down. Once the paper is dry, it may be pressed flat with heavy books.
• The sun print paper reacts with ultraviolet light. Any reaction in an area of the spectrum other than ultraviolet is due to light reflection causing a scattering of ultraviolet rays.
• Have students research applications for infrared and ultraviolet technology.
• This demonstration can easily be conducted as a student laboratory activity with additional materials available from Flinn Scientific—glass prisms (Catalog No. AP9237), thermometers (Catalog No. AP5406) and sun print paper (Catalog No. FB1554).

Sample Data

Data Table A

Data Table B

Answers to Questions

1. Calculate the change in temperature (ΔT) for the Herschel experiment over 5 minutes for each thermometer by subtracting the initial temperature (0 minutes) from the final temperature (5 minutes). Record the difference in Data Table B.
2. Describe the change in temperature over time for each of the thermometers. Which thermometer had the highest temperature?

   The shade thermometer reading only increased slightly, followed by blue with an increase of 3 degrees. The temperature of the yellow area of the spectrum increased 5 degrees. The thermometer in the area beyond the red end of the spectrum reached the highest temperature, resulting in the greatest increase in temperature, 8 °C.

3. What can you infer about the region beyond the red end of the visible spectrum?

   Some form of energy must exist beyond visible light since the greatest increase in temperature was observed beyond the red.

4. In the following space, draw the outline of the visible spectrum from the Ritter Experiment, marking the red end and the violet end. Shade in the darkened area as it appeared on the exposed sun print paper.
   {12757_Answers_Figure_8}
5. What can you infer about the region beyond the violet end of the visible spectrum? What evidence led you to this conclusion?

   Some form of energy must exist beyond the violet end of visible light. The most intense reaction on the sun print paper was on either side of the violet end, extending past the spectrum. Almost no reaction was observed at the red end of the spectrum.

6. (Optional) Herschel most likely used the Fahrenheit temperature scale. Convert the temperature change from Celsius to Fahrenheit degrees by using Equation 1. Record the Fahrenheit temperature change for each thermometer in the following space.
   {12757_Answers_Equation_1}
   Shade: Δ°F = 9/5 (1 Δ°C) = 1.8 °F, Blue = 5.4 °F, Yellow = 9 °F, Infrared = 14.4 °F

References

Getting Hotter? http://stargazers.gfsc.nasa.gov (Accessed May 2008).

Herschel Infrared Experiment, http://coolcosmos.ipac.caltech.edu (Accessed May 2008).

Mystery Light, http://stargazers.gfsc.nasa.gov (Accessed May 2008).

The Ritter Experiment, http://coolcosmos.ipac.caltech.edu (Accessed May 2008).