William Herschel

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Recorder – records the temperatures on the observation chart

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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.

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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.

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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).

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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).

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Herschel Experiment Ideas

Makes a great science fair activity:

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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?

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Infrared Measures Heat