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PROFESSOR: Invisible light.

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And Steve, can you say
your definition again?

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AUDIENCE: Light that cannot
be detected by the human eye.

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PROFESSOR: Light that cannot
be detected by the human eye.

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OK, so just because we
can't detect that light

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doesn't mean that
there's no light there.

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So how do we detect these
other kinds of light?

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I know that you guys
read about visible light,

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you read about
ultraviolet light,

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you read about X-ray light.

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And one thing that I'm going
to say in this article,

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they go back and forth
between light and radiation,

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electromagnetic radiation.

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Whenever we talk about these
different kinds of light,

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I always want you
to call it "light."

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So it's radio light, or infrared
light, X-ray light, gamma ray

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light, just to kind
of keep things clear.

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Because any of the measurements
that we did with light up

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until now, we measured its
color, we measured its flux,

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we measured a flux of a
certain kind of light,

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we measured luminosity.

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Now we can do all of
those same measurements,

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but we can do them with
different kinds of light.

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We said before, if
we had a detector,

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our detector could
detect visible light.

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If we put a filter in front,
and that was a red filter,

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we'd only let red light through.

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We'd only let photons that
have an energy that we see

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or recognized as red through.

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So this is how it works
for visible light.

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In the case of other
objects, in the case

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of other kinds of light,
if we're looking at objects

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out in the solar system
that are giving off,

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say, infrared light, if we
looked with our detector,

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even if we took the
red filter away,

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our detector is not going
to be sensitive to that kind

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of light.

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So for us, we were able
to look at visible light

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and we broke it up into
red, green, and blue.

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When we're looking at
other kinds of light,

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we can't use the same
kind of detector.

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We can't use the
same kind of camera.

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We have to have a camera that
is sensitive to visible light,

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or X-ray light, or radio light.

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And it's not that we
have the same detector

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and we just put an
infrared filter in front,

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or an ultraviolet
filter in front,

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or an X-ray filter in front.

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We could do that if
we had a detector that

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was sensitive to all of
those kinds of light,

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but each different kind of
light has its own special kind

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of detector.

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So we're going to say that
invisible light, let's say,

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must be detected with
a special detector

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for each type of light.

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We're going to look at this
a little bit more tomorrow,

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but we're going to look right
now at invisible light that

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was detected by the
Chandra X-ray Observatory.

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Tomorrow morning I'm going to
have you read a little bit more

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about the Chandra
X-ray Observatory

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so you can see the
instrument itself.

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The Chandra X-ray
Observatory detects x-rays.

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If we took a flashlight and we
shined it inside the telescope,

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it wouldn't see anything.

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It's not sensitive
to visible light.

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We're also going to look
today at images or data taken

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by the Hubble Space Telescope--

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and that detects visible light,
but they're slightly different.

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We're going to see that the
Hubble Space Telescope, just

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like our other detectors, only
detects the number of photons

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that come in.

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We have to put a
filter in front if we

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want to learn about red, green,
or blue light that's coming in.

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AUDIENCE: But is it
possible to do that?

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Can the Hubble
telescope do that?

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PROFESSOR: Can the
Hubble Telescope

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make red, green and
blue filtered images?

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Yeah.

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So in front of most
visible light telescopes,

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there's a little wheel that
has different filters on it,

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and it drops a red
filter in front

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and then the camera
takes a picture

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of whatever you're looking at.

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And then you drop a
blue filter in front,

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you take another picture.

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You drop the green
filter in front,

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you take another picture.

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So even out there in
outer space there's

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these filters that drop
down in front of the images,

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and that's how we end
up with these true color

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images from the Hubble
Space Telescope.

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And you guys will see
some of these in the press

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releases that you're using
for your expert projects.

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So we're going to talk a
little bit tomorrow also

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about how the Chandra
X-ray Observatory collects

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different information.

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It doesn't just collect
a number of photons,

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it actually does
collect information

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about the energy of each photon.

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So we don't have to use
filters on the Chandra X-ray

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Observatory.

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But for right now what we're
going to do, in the same way

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that you guys were
looking at these images,

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these images are all
of the Crab Nebula,

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but they're images
of the Crab Nebula

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that were observed by
different telescope.