December 10, 2019 1

The Physics Show – Infrared Light, Astronomy and SOFIA

The Physics Show – Infrared Light, Astronomy and SOFIA


Welcome, you are watching a Physics Show video! We are going to show you a neat demonstration
using infrared light. We have a heating element, like the one on an electric stove, or in a
toaster, in front of a special mirror. Now we’ll swing around our equipment to show
you a second mirror. In front of this second mirror is a bundle of matches, and we’ll use
heat in the form of light, generated by the heating element in front of the first mirror,
to set the matches on fire. The first question you can ask is “How can
light set something on fire, don’t you need heat?” Well, if you step out into sunlight,
you’ll quickly remember that light does carry heat. The next thing we need to understand is the
mirrors. They are in a special shape called a parabola. A feature of a parabola is that
any line starting at a place called the focal point will be reflected straight outwards
by the mirror. This is how flashlights work. In our case, the heat from the element is
sent in one direction. When the light hits the second mirror, all of this happens in
reverse, and the heat is concentrated on the matches in the focal point. This is similar
to using a magnifying glass to start a fire. Looking at the matches, we can see the red
glow of the heating element reflected in the mirror behind it. But this is just a small
circle of light, it turns out that most of the heat is invisible, it’s called infrared
light, and we’ll talk about that in a minute. I can see smoke, if we wait a little while
longer, the matches will burst into flames. That was really neat, we were able to set
those matches on fire using only reflected light. Now we’re going to pull back the camera
so you can see how far apart these two mirrors were. We set the matches on fire from very
far away. We need to talk more about infrared light.
Sir William Hershel knew that sunlight could make you feel warm. He also knew that you
could use a prism to divide light into the colors of the rainbow. He took a thermometer
and placed it into each of the different colors, measuring the change in temperature compared
to thermometers far away from the light. He made an amazing discovery, the thermometer
got warm in a band beyond red light. This meant there was a color our eyes could not
see, and this invisible light still carried heat. He named this color infrared, which
means “below red”, since it was just below red in the spectrum. But Hershel did more than just this. He, along
with his sister Caroline, was a great astronomer. Today most people know him as the person who
discovered the planet Uranus, but many astronomer think that his discovery of infrared light
is just as important as a planet. Astronomers like using infrared light because
it is connected to temperature. In the picture to the left we see the constellation Orion
in the visible spectrum. Rigel is in blue, which means it is very hot. Betelgeuse is
in red, which means that it is cooler. But both are stars, so they are very hot indeed.
If we look at the same patch of sky in the infrared, we can see cooler objects.
Many of these are clouds of dust and gas, where stars are just being born. They aren’t
hot enough to shine with the light our eyes can see. Witnessing the birth of new stars
is just one of the advantages of using infrared light in astronomy. Infrared astronomy has a big problem. Infrared
light is blocked by our atmosphere. This picture tells the story. Our atmosphere lets through
visible light, and also radio and television. But infrared light can’t get through. This
is mainly due to water vapor in our atmosphere. If only there was a way to get above all of
that water vapor… The Stratospheric Observatory for Infrared
Astronomy, SOFIA for short, is operated by NASA and the German Space Program. It is an
infrared telescope with a 100 inch mirror mounted on a 747 airplane. When SOFIA gets above the water in our atmosphere,
a door opens at the back of the plane for the infrared telescope. Let’s look at the insides of SOFIA. The people
running the telescope sit in the front. The telescope is in the back. The telescope’s
job is to collect infrared light, which is then sent to a scientific instrument. A key
feature of SOFIA is that these instruments can be taken out and replaced by other instruments,
if scientists want to look at different things in the infrared light. This is a flexibility
we don’t have with space-based telescopes, and also allow for repairs and
upgrades. Another advantage of an airplane is that it
can fly to many different places. In 2013, SOFIA was sent to New Zealand, to observe
constellations in the Southern Sky. These are best viewed from the Southern Hemisphere,
ground-based telescopes can’t move like SOFIA. SOFIA is able to move to specific locations
to observe events like eclipses and comets that could not be seen from other locations. SOFIA is an observatory with many unique abilities
that will help advance our knowledge of our universe. We hope you’ve enjoyed learning about infrared
light, astronomy, and the SOFIA program. We’ll see you at the next Physics Show!

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