Herschel Experiment

1. Herschel Experiment

2. The Herschel Experiment: A Classroom Activity Objective: To perform a version of the experiment of 1800, in which a form of radiation other than visible light was discovered by the famous astronomer Sir Frederick William Herschel. Target Audience: Grades 7-9, easily extended to higher grades (10-12)

3. Herschel Experiment Set-Up • Place a sheet of white paper inside a cardboard box • Tape three thermometers together and place inside box • Cut a small notch in the top of the box and position a small glass prism so that the spectrum is projected inside the box • Arrange the thermometers so that one is just outside the red end of the spectrum, with no visible light falling on it Do not look directly at the Sun.

4. Results The thermometers will register differences almost immediately. After approx. 10 minutes, the thermometers show a 10 degree difference: The highest temperature is found outside the visible spectrum, where infrared light is falling on the thermometers.

5. Team Roles The Herschel Experiment can be done in groups of 2 to 4. Team members can be assigned the following roles: • Box-keeper – makes sure the thermometers are positioned properly and holds box if necessary • Timekeeper – announces times for temperature measurements • Measurer – reads the temperatures of the thermometers • Recorder – records the temperatures on the observation chart

6. Today’s Goals • Adjust the prism in the notch until a broad spectrum is achieved. • Place thermometers in the blue, the yellow, and next to the red portions of the spectrum for 30 to 60 seconds, until a temperature difference is observed. • If 3 thermometers will not fit in the spectrum, place one thermometer in the yellow and one thermometer next to the red portion of the spectrum. Optional: Keep the 3rd thermometer in a shaded area of the box, as a control. • Using the Herschel Experiment Worksheet, continue taking data for 4 to 5 minutes.

7. Workshop Discussion Useful discussion questions for the workshop (and classroom) setting: • Did you observe a temperature rise in the infrared? • Did you encounter any difficulties while doing the experiment? If so, how did you solve them? • Consider recording the final thermometer readings and elapsed time for each group. Comparing results can lead to an interesting discussion of experimental uncertainties.

8. Additional Questions • Insightful students may wonder why the blue thermometer doesn’t have the highest temperature. After all, doesn’t blue light have more energy than red light? • The answer lies in what the prism does to the light. Prisms spread blue light out, while red light remains more focused. After passing through the prism, the energy in the red and infrared parts of the spectrum is more concentrated. • A more thorough discussion can be found on the SIRTF website (sirtf.caltech.edu).

9. Helpful Hints For best results, do experiment in direct sunshine, when the Sun is high (noon). Putting the prism in the cut notch provides stability and easy rotation. Glass prisms work much better than plastic ones (this is Herschel’s original prism). Blacken the bulbs of the thermometers with black paint. Flat black spray paint works best, but even a black magic marker will yield results. The experiment WILL NOT WORK without the paint (Herschel soot-blackened his thermometers in the original experiment).

10. Herschel Experiment Ideas • Makes a great science fair activity:

11. The Importance of Infrared • How is infrared light used in our daily lives? • Why would astronomers put a telescope in space to look at infrared light?

12. Infrared Measures Heat Every object with a temperature above absolute zero radiates in the infrared.