An activity you can use in the classroom
When sunlight falls on the eyes, the brain registers
it as a visible image. But there is an invisible part
that we miss because human vision is sensitive only
to a narrow range of wavelengths. In the 18th century,
astronomers William Herschel and Johann Ritter proved
the existence of invisible waves of sunlight. Herschel
proved the existence of infrared waves, and Ritter proved
the existence of ultraviolet rays. The only way to detect this invisible radiation is
to use special instruments. The spectrometer collects
different wavelengths of light and analyzes them. In
this activity, you'll reenact Herschel's and Ritter's
experiments, seeing for yourself evidence that radiant
energy exists above and below the visible portion of
the spectrum. 
- prism
- light source
- three weather thermometers
- pencil and felt-tip pen
- several sheets of blueprint paper
- one quart (about 1 liter) of household ammonia
- flat pan
- Start with William Herschel's
experiment. Allow the three thermometers to register
the air temperature where the experiment will be done
for about five minutes. Carefully note the temperatures
and record them on the attachedworksheet.
- Using the prism, create
a spectrum with sunlight as the source.
- Place the first thermometers
in the violet range of the spectrum, the second thermometer
in the spectrum's center, and the third thermometer
barely beyond the spectrum's red end.
- Leave the thermometers
in the spectrum for at least five minutes, moving
them carefully as the sunlight moves the spectrum.
Temperature changes may be slight, so observe carefully.
- On the worksheet, record
the final temperature readings in the chart, and answer
questions 2 and 3.
- Now re-create Johann Ritter's
experiment. Using sunlight from an open window and
the prism, create a spectrum on a horizontal surface.
(Glass blocks most ultraviolet light.) Make sure the
prism is resting on a stable object so that it won't
move.
- Working quickly so that
the paper is not exposed to too much light, cut a
piece of blueprint paper about four times larger than
the spectrum.
- Place the paper beneath
the spectrum. Then, outline the area covered by the
spectrum with a felt-tip pen and label the violet
end.
- Put just enough household
ammonia in the bottom of the pan to cover the bottom
to a depth about 0.5 inches (1 cm). Place the pan
near an open window.
- Take the blueprint paper
from underneath the prism, and hold it over the ammonia
so the fumes process the paper.
- Answer questions 4 and
5 on the worksheet.
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