The following description
of the experimental setup is borrowed (with kind permission) from
NASA's
CoolCosmos website. It also provides a guide for teachers
on the experiment.
In the year 1800, Sir William Herschel discovered the existence
of infrared in an astronomical source by performing an experiment
very similar to the one we show here. Herschel passed sunlight
through a prism. As sunlight passes through the prism, the prism
divides it into a rainbow of colors called a spectrum. A spectrum
contains all of the colors which make up sunlight. Herschel was
interested to find out whether the sun would also give off a kind
of invisible radiation that could be felt as heat.
To
do this he used thermometers with blackened bulbs and measured
the temperature of the different colors of the spectrum. He noticed
that the temperature increased from the blue to the red part of
the spectrum. Then he placed a thermometer just past the red part
of the spectrum in a region where there was no visible light and
found that the temperature there was even higher. Herschel realized
that there must be another type of light which we cannot see in
this region. This light is now called “infrared”.
It
is quite easy to recreate the Herschel experiment. All you need
is a glass prism, three alcohol thermometers, a white piece of
paper and a box. Paint the thermometer bulbs with black paint
to make this experiment work. The experiment must be conducted
outdoors on a sunny day.
Begin
by placing the white sheet of paper flat in the bottom of the
cardboard box. Carefully attach the glass prism near the top edge
of the box on a cutout notch. It should hold the prism snugly,
while permitting the prism’s rotation about its long axis.
After the prism is secured in the notch, place the thermometers
in the shade and record the ambient air temperature.
Then
place the thermometers in the spectrum such that one of
the bulbs is in the blue region, another is just beyond
the (visible) red region, and the third remains in the shade.
It will take about five minutes for the temperatures to
reach their final values. Record the temperatures in each
of the three regions: shade, blue, and "just beyond" the
red. Do not remove the thermometers or block the light while
reading the temperatures.
What do you expect to happen? Which thermometers will rise
with time?
Some
quick calculations:
1. What is the difference between the final temperature readings
from the thermometer in the shade and the one in the blue?
2. What is the difference between the final temperature readings
from the thermometer in the shade and the one just beyond the
red?
What are your results? What do
you notice about your temperature readings? Do you see any differences
between the thermometer just beyond the red and the one in the
shade? If so, where does that difference come from? After all,
in both cases, you cannot see any light on the thermometers!
We would love to hear about your results. Send us your initial
and final temperature readings and we will post them to show
everybody that, after more than 200 years, William Herschel's
experiment still works!
