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MARK HARTMAN: Remember
that every detector is not

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00:00:24,060 --> 00:00:25,770
sensitive to every
kind of light.

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00:00:25,770 --> 00:00:30,450
In particular, when I shine
this remote control at you,

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00:00:30,450 --> 00:00:33,630
you don't see the
light because your eyes

12
00:00:33,630 --> 00:00:36,690
aren't sensitive to the
photons that this emitter is

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00:00:36,690 --> 00:00:37,260
putting out.

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00:00:37,260 --> 00:00:38,718
This emitter is
putting out photons

15
00:00:38,718 --> 00:00:41,010
that are a little bit less
than two electron volts--

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00:00:41,010 --> 00:00:43,770
well, I think a little bit
less than 1.8 electron volts,

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00:00:43,770 --> 00:00:47,146
and your eyes don't
pick up that energy.

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00:00:47,146 --> 00:00:49,020
When those photons hit
the back of your eyes,

19
00:00:49,020 --> 00:00:51,100
it doesn't send a little
signal to your brain,

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00:00:51,100 --> 00:00:56,274
but if I shine this
at our projector,

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00:00:56,274 --> 00:00:57,690
if you look on the
screen, you can

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00:00:57,690 --> 00:01:02,130
see that we can tell that there
is a little light flashing.

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00:01:02,130 --> 00:01:04,800
In that case, that detector
is sensitive to photons that

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00:01:04,800 --> 00:01:07,300
have that low of an energy.

25
00:01:07,300 --> 00:01:11,940
However, if I were to look at
something with a telescope,

26
00:01:11,940 --> 00:01:15,210
I'm not always going to get
the same number of photons

27
00:01:15,210 --> 00:01:16,770
that my source puts out.

28
00:01:16,770 --> 00:01:19,650
I'm putting out photons
here, but when I look at it,

29
00:01:19,650 --> 00:01:21,480
I'm not collecting that.

30
00:01:21,480 --> 00:01:23,562
It's not that
there's no flux there

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00:01:23,562 --> 00:01:25,020
or that there's no
flux to collect.

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00:01:25,020 --> 00:01:26,760
I'm just not sensitive to it.

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00:01:26,760 --> 00:01:29,070
So we are going to put
together a little model that's

34
00:01:29,070 --> 00:01:30,900
going to help us
answer this question--

35
00:01:34,020 --> 00:01:36,360
how is-- and again,
I want you to write

36
00:01:36,360 --> 00:01:37,950
a couple of these notes down.

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00:01:37,950 --> 00:01:43,890
So today is the 11th,
and it is about 10:10.

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00:01:43,890 --> 00:02:05,170
How is the light we
record different from what

39
00:02:05,170 --> 00:02:06,685
the source emits?

40
00:02:16,080 --> 00:02:17,880
And we're going to
draw a little diagram.

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00:02:17,880 --> 00:02:19,760
Again, we're going to have--

42
00:02:19,760 --> 00:02:22,740
and here's our source.

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00:02:22,740 --> 00:02:25,230
It could be a cloud like we
said with the Orion nebula.

44
00:02:25,230 --> 00:02:27,570
It could be some stars.

45
00:02:27,570 --> 00:02:30,840
It's putting out light
in all directions.

46
00:02:35,350 --> 00:02:38,920
Some of that light
happens to be moving

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00:02:38,920 --> 00:02:43,072
towards us, and over here--

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00:02:43,072 --> 00:02:44,905
I want you to leave
some space in the middle

49
00:02:44,905 --> 00:02:47,180
because we're going to come
back to this in a second.

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00:02:47,180 --> 00:02:48,596
Over here, we've
got our detector.

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00:02:53,970 --> 00:02:57,039
It's this grid of pixels.

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00:02:57,039 --> 00:02:59,580
We said that the detectors are
at the back of our telescopes.

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00:02:59,580 --> 00:03:03,150
So our telescope is going to
collect some of this light.

54
00:03:03,150 --> 00:03:05,680
It's going to keep
going this way.

55
00:03:09,880 --> 00:03:13,600
Some of it is going to
get here to our detector.

56
00:03:13,600 --> 00:03:19,090
Now, if we were to grab all
the light from right here--

57
00:03:19,090 --> 00:03:20,460
if we were to take this light--

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00:03:23,310 --> 00:03:27,300
if we were to take all the
light that we could just

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grab from right there,
and if we could somehow

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00:03:30,780 --> 00:03:39,650
grab that and look at it and we
wanted to look at its spectrum,

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00:03:39,650 --> 00:03:40,910
we look at the intensity.

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00:03:40,910 --> 00:03:43,310
We look at a bar chart of
intensity versus energy.

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And in particular, we could
say we'd look at red, yellow,

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00:03:51,330 --> 00:03:52,700
green, and blue.

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00:04:01,710 --> 00:04:07,380
This histogram tells
us what kind of light

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is the source giving
out, but the only thing

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00:04:10,800 --> 00:04:14,860
that we can measure is
what our detector tells us.

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00:04:14,860 --> 00:04:18,880
So from the
detector, we actually

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would get a different
intensity versus energy graph--

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red, yellow, green, and blue.

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What we're going to do--

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00:04:32,400 --> 00:04:34,231
we are going to represent--

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we're going to model.

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00:04:37,680 --> 00:04:48,580
We're going to make a
model, and each group

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is one part of the
source or detector.

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00:05:00,180 --> 00:05:04,370
So half of us are
going to be the source.

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We're going to be one
part of the source,

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and we're going
to give out light.

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And then one part
of us is going to be

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00:05:14,570 --> 00:05:18,620
one pixel of the detector.

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We're going to look
at a thing called

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detector response, and what
detector response means is

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00:05:41,630 --> 00:05:51,650
we record only
part of the light,

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00:05:51,650 --> 00:05:54,960
and that depends on what the
energy of the photons are.

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So here we have--

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I've reversed them.

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So here we have the source
from our flat spectrum.

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00:06:09,860 --> 00:06:12,050
This is what the
spectrum looks like when

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00:06:12,050 --> 00:06:13,310
the source produced it.

90
00:06:13,310 --> 00:06:14,390
It's flat.

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00:06:14,390 --> 00:06:18,260
It's just bars that go straight
across, but our detector--

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00:06:18,260 --> 00:06:23,595
what is different about our
detector, what it shows to us?

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00:06:23,595 --> 00:06:25,050
AUDIENCE: Took out some.

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00:06:25,050 --> 00:06:26,258
MARK HARTMAN: Say that again.

95
00:06:26,258 --> 00:06:28,084
AUDIENCE: Took out
some of the photons.

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00:06:28,084 --> 00:06:30,000
MARK HARTMAN: It took
out some of the photons?

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00:06:30,000 --> 00:06:32,047
AUDIENCE: It didn't
collect all the photons.

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00:06:32,047 --> 00:06:33,880
MARK HARTMAN: It didn't
detect, or it didn't

99
00:06:33,880 --> 00:06:36,160
collect all of the photons.

100
00:06:36,160 --> 00:06:38,680
They were there, but
the detector wasn't--

101
00:06:38,680 --> 00:06:42,790
it's not as sensitive to blue
light as it is to red light.

102
00:06:42,790 --> 00:06:44,470
So even though the
source gave out

103
00:06:44,470 --> 00:06:46,510
a spectrum that
looked like this,

104
00:06:46,510 --> 00:06:49,840
the detector gives
us in our measurement

105
00:06:49,840 --> 00:06:52,370
a spectrum that looks like that.

106
00:06:52,370 --> 00:06:53,800
So let me ask you this.

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00:06:53,800 --> 00:06:55,300
What if I had--

108
00:06:57,880 --> 00:07:00,490
in the same way, this is
the response of our eyes.

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00:07:00,490 --> 00:07:03,080
Our eyes are less
sensitive to blue light.

110
00:07:03,080 --> 00:07:05,650
So if we had a spectrum
where the same amount of blue

111
00:07:05,650 --> 00:07:09,280
was put out, it would look
like a lower intensity to us.

112
00:07:09,280 --> 00:07:12,220
I want you to just
give a prediction,

113
00:07:12,220 --> 00:07:15,400
and I just want you to sketch
in your notebook what would

114
00:07:15,400 --> 00:07:19,510
a flat spectrum look like if,
instead of having this detector

115
00:07:19,510 --> 00:07:21,760
response, maybe we had a
different detector that

116
00:07:21,760 --> 00:07:23,680
was made out of a
different material?

117
00:07:23,680 --> 00:07:27,640
And instead, we kept
two out of five red,

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00:07:27,640 --> 00:07:33,160
three out of five yellow,
four out of five green,

119
00:07:33,160 --> 00:07:35,170
and five out of five blue?

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00:07:35,170 --> 00:07:37,450
Say our detector was
sensitive to blue,

121
00:07:37,450 --> 00:07:39,430
and it detected
everything, but it

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00:07:39,430 --> 00:07:41,570
wasn't very sensitive to red.

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00:07:41,570 --> 00:07:46,240
So in this case, let's look
at what we came out with.

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00:07:46,240 --> 00:07:49,302
What is different
between these two graphs?

125
00:07:49,302 --> 00:07:51,110
AUDIENCE: [INAUDIBLE].

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00:07:51,110 --> 00:07:53,525
MARK HARTMAN: You may have
to squeeze over a little bit.

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00:07:53,525 --> 00:07:55,765
AUDIENCE: [INAUDIBLE].

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00:07:55,765 --> 00:07:57,390
MARK HARTMAN: So how
would you describe

129
00:07:57,390 --> 00:08:00,430
the difference between
these two energy histograms

130
00:08:00,430 --> 00:08:02,040
or spectrums--

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00:08:02,040 --> 00:08:03,534
spectra?

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00:08:03,534 --> 00:08:04,469
AUDIENCE: Spectra.

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00:08:04,469 --> 00:08:06,510
MARK HARTMAN: It's giving
us the number of counts

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00:08:06,510 --> 00:08:11,280
in one second that were
emitted, and then this

135
00:08:11,280 --> 00:08:13,260
is the number of
counts in one second

136
00:08:13,260 --> 00:08:15,450
that we collected
at our detector.

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00:08:15,450 --> 00:08:16,980
We didn't collect all of them.

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00:08:16,980 --> 00:08:19,480
So how would you describe the
difference between these using

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00:08:19,480 --> 00:08:21,230
the words intensity and energy?

140
00:08:24,194 --> 00:08:26,955
AUDIENCE: The intensity of
the first one is higher.

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00:08:26,955 --> 00:08:28,580
MARK HARTMAN: The
intensity of this one

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00:08:28,580 --> 00:08:30,530
is higher because
the bars are higher.

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00:08:30,530 --> 00:08:32,850
The y-axis measures in density.

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00:08:32,850 --> 00:08:33,650
So what we detect--

145
00:08:33,650 --> 00:08:36,770
we detect less intensity.

146
00:08:36,770 --> 00:08:38,139
What else?

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00:08:38,139 --> 00:08:40,610
AUDIENCE: And there was more
energy in the first one,

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00:08:40,610 --> 00:08:43,610
and there's less
energy in the second.

149
00:08:43,610 --> 00:08:46,550
We collected less energy.

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00:08:46,550 --> 00:08:48,768
MARK HARTMAN: Which axis
here has to do with energy?

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00:08:48,768 --> 00:08:52,050
AUDIENCE: The one
with the photons.

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00:08:52,050 --> 00:08:53,550
MARK HARTMAN: The x-axis, right?

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00:08:53,550 --> 00:08:57,400
So I think you're saying here at
high energies, or blue photons,

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00:08:57,400 --> 00:08:59,590
there was a lot
that were put out,

155
00:08:59,590 --> 00:09:03,270
but we didn't actually
collect a whole lot.

156
00:09:03,270 --> 00:09:05,370
Is that what you're
trying to say?

157
00:09:05,370 --> 00:09:06,150
What else?

158
00:09:06,150 --> 00:09:06,886
Azeith?

159
00:09:06,886 --> 00:09:09,562
AUDIENCE: The intensity is
low for yellow and green.

160
00:09:09,562 --> 00:09:13,450
When it started off,
was high, and when

161
00:09:13,450 --> 00:09:17,350
it got to the detector,
it equaled out.

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00:09:17,350 --> 00:09:20,500
MARK HARTMAN: So even though
our source put out more yellow--

163
00:09:20,500 --> 00:09:22,870
or, I'm sorry-- put
out more green--

164
00:09:22,870 --> 00:09:27,220
or put out more blue than green
and yellow, what we detect

165
00:09:27,220 --> 00:09:31,000
is that it actually collects
or records a little bit more

166
00:09:31,000 --> 00:09:33,700
green and yellow
than it does blue.

167
00:09:33,700 --> 00:09:34,570
Well, that's great.

168
00:09:34,570 --> 00:09:37,390
Doesn't that screw us?

169
00:09:37,390 --> 00:09:39,850
If we're interested in
what's physically going on

170
00:09:39,850 --> 00:09:42,940
at the object, but
we record this,

171
00:09:42,940 --> 00:09:46,012
how is that useful at all?

172
00:09:46,012 --> 00:09:47,470
How can we get
around this problem?

173
00:09:52,500 --> 00:09:54,098
David, what do you think?

174
00:09:54,098 --> 00:09:56,650
AUDIENCE: The ratio
is from what you

175
00:09:56,650 --> 00:10:03,040
have to receive to get the
source's actual emissions.

176
00:10:03,040 --> 00:10:06,070
MARK HARTMAN: If I know
how my detector responds,

177
00:10:06,070 --> 00:10:09,460
if I know that it only collects
two out of five or five

178
00:10:09,460 --> 00:10:12,550
out of five, I can
take my measurements,

179
00:10:12,550 --> 00:10:14,710
and I can kind of turn
it back into this.

180
00:10:17,350 --> 00:10:20,920
So I have to know exactly
what my detector is doing

181
00:10:20,920 --> 00:10:23,860
to be able to then reconstruct
or figure out what's really

182
00:10:23,860 --> 00:10:25,090
going on over here.

183
00:10:28,000 --> 00:10:32,380
So I want you guys to take
a break, and I want you to--

184
00:10:32,380 --> 00:10:34,970
For each group, you
guys are the source,

185
00:10:34,970 --> 00:10:38,930
you guys are the cloud,
you guys are the detector.

186
00:10:38,930 --> 00:10:42,100
So I want you to
figure out who's

187
00:10:42,100 --> 00:10:44,679
going to be responsible for
red light, red photons, who's

188
00:10:44,679 --> 00:10:46,720
going to be responsible
for yellow photons, who's

189
00:10:46,720 --> 00:10:48,180
going to be
responsible for green,

190
00:10:48,180 --> 00:10:49,971
and who's going to be
responsible for blue.

191
00:10:53,064 --> 00:10:53,980
Same thing over there.

192
00:10:56,530 --> 00:10:58,830
And then I want you to
add this to your notes.

193
00:11:02,166 --> 00:11:07,076
[SIDE CONVERSATION]

194
00:11:22,810 --> 00:11:24,940
So I want you guys to
add this onto your notes.

195
00:11:24,940 --> 00:11:30,256
We're going to put on
interstellar material.

196
00:11:35,660 --> 00:11:38,840
Interstellar just means
in between the stars.

197
00:11:38,840 --> 00:11:40,340
So if we're looking
at another star,

198
00:11:40,340 --> 00:11:42,410
we're looking at
interstellar material.

199
00:11:42,410 --> 00:11:44,400
If you're looking
at another galaxy,

200
00:11:44,400 --> 00:11:47,250
you may be looking at
intergalactic material,

201
00:11:47,250 --> 00:11:49,850
but we've still got our source.

202
00:11:49,850 --> 00:11:52,830
It's still sending out
photons in all directions.

203
00:11:52,830 --> 00:11:56,960
The photons move this
way, some of them,

204
00:11:56,960 --> 00:11:59,990
and we're looking first at the--

205
00:11:59,990 --> 00:12:03,170
if we were to grab these photons
and look at the intensity

206
00:12:03,170 --> 00:12:04,340
right there.

207
00:12:04,340 --> 00:12:06,050
Then we've got some
photons that go

208
00:12:06,050 --> 00:12:10,625
through here, some photons
that get stopped here.

209
00:12:21,620 --> 00:12:23,450
So some photons
get stopped there.

210
00:12:23,450 --> 00:12:27,890
So now we're going to
actually put in another graph,

211
00:12:27,890 --> 00:12:32,080
and we're going to say, let's
look what do these photons--

212
00:12:38,590 --> 00:12:40,841
so first we're going
to look at the source.

213
00:12:40,841 --> 00:12:42,340
Then we're going
to look at what the

214
00:12:42,340 --> 00:12:45,490
photons after they've
passed through the cloud--

215
00:12:45,490 --> 00:12:50,680
what do those look
like in intensity

216
00:12:50,680 --> 00:12:52,300
as a function of energy?

217
00:12:52,300 --> 00:12:54,760
And then those-- the cloud--

218
00:12:54,760 --> 00:12:57,700
that light gets to the
detector, and then the detector

219
00:12:57,700 --> 00:13:00,197
is going to say, all
right, what do I detect?

220
00:13:00,197 --> 00:13:01,780
So in each of these
cases, we're going

221
00:13:01,780 --> 00:13:04,510
to look at what is the
spectrum of the light

222
00:13:04,510 --> 00:13:07,240
just after the source,
just after the interstellar

223
00:13:07,240 --> 00:13:09,825
material, and then by the
time it gets to the detector.

224
00:13:12,630 --> 00:13:15,310
So I know you guys are in
the middle of predicting,

225
00:13:15,310 --> 00:13:20,060
but let's take a look
at what we actually had.

226
00:13:20,060 --> 00:13:22,550
So if everybody needs to
scoot in a little bit,

227
00:13:22,550 --> 00:13:26,510
here we had our source
from our flat spectrum.

228
00:13:26,510 --> 00:13:29,720
We had 20 photons of each color.

229
00:13:29,720 --> 00:13:33,355
What happened after it
went through the cloud?

230
00:13:36,240 --> 00:13:40,640
Oh, I wanted us to keep the
same scale, but we'll see.

231
00:13:40,640 --> 00:13:43,106
So what happened when it
went through the cloud?

232
00:13:43,106 --> 00:13:44,600
AUDIENCE: Which one's the cloud?

233
00:13:44,600 --> 00:13:45,475
AUDIENCE: The middle.

234
00:13:45,475 --> 00:13:47,050
MARK HARTMAN: So this is just--

235
00:13:47,050 --> 00:13:50,410
so this represents
this light that

236
00:13:50,410 --> 00:13:51,940
just came out of the source.

237
00:13:51,940 --> 00:13:57,280
The middle represents the light
after it came out of the cloud,

238
00:13:57,280 --> 00:13:59,790
and then the right hand
one represents the light

239
00:13:59,790 --> 00:14:01,540
that we actually record
from the detector.

240
00:14:05,110 --> 00:14:06,590
So in this case--

241
00:14:06,590 --> 00:14:07,090
good.

242
00:14:07,090 --> 00:14:07,840
Nice.

243
00:14:07,840 --> 00:14:09,340
Good work.

244
00:14:09,340 --> 00:14:14,950
So what happened to the light
that came from the source?

245
00:14:14,950 --> 00:14:19,170
After it went through the
cloud, how was it different?

246
00:14:19,170 --> 00:14:22,960
AUDIENCE: The could collects
some photons from the source.

247
00:14:22,960 --> 00:14:25,980
MARK HARTMAN: Use the word
energy in that sentence.

248
00:14:25,980 --> 00:14:32,920
AUDIENCE: The cloud collects
some energy from the source.

249
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MARK HARTMAN: Does it collect
energy as red photons?

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00:14:37,390 --> 00:14:41,970
Does it stop photons with
energies that we see as red?

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00:14:41,970 --> 00:14:43,170
AUDIENCE: No.

252
00:14:43,170 --> 00:14:47,770
It only collects green and blue.

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00:14:47,770 --> 00:14:52,060
MARK HARTMAN: So the
cloud collects or stops

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00:14:52,060 --> 00:14:53,260
some green and some blue.

255
00:14:53,260 --> 00:14:55,870
This is what was let through.

256
00:14:55,870 --> 00:14:59,230
So we started off with all the
same, but when we let through,

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00:14:59,230 --> 00:15:02,320
we see that blue is
let through less.

258
00:15:02,320 --> 00:15:06,260
So we went from 20 photons
per second down to--

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00:15:06,260 --> 00:15:07,400
what is this, 12?

260
00:15:07,400 --> 00:15:10,480
12 photons per second.

261
00:15:10,480 --> 00:15:13,480
So then, of course, we
can't go out in space

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00:15:13,480 --> 00:15:14,530
and grab that light.

263
00:15:14,530 --> 00:15:19,390
We still have to grab what
we see from the detector.

264
00:15:19,390 --> 00:15:22,570
Now, what is the difference
from the detector?

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00:15:22,570 --> 00:15:25,480
It's hard to tell because
they expanded their scale

266
00:15:25,480 --> 00:15:29,530
and put 0 to 20 all the way
up here, but let's compare.

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00:15:29,530 --> 00:15:36,940
We've got 20 counts of red
that we got through the cloud,

268
00:15:36,940 --> 00:15:40,420
but we detect 20 counts of red.

269
00:15:40,420 --> 00:15:44,200
Then we only detect
12 counts of yellow,

270
00:15:44,200 --> 00:15:47,020
but there were 20
counts of yellow before.

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00:15:47,020 --> 00:15:48,950
So what happened to some
of the yellow light?

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00:15:48,950 --> 00:15:50,420
AUDIENCE: It got stopped.

273
00:15:50,420 --> 00:15:53,850
It didn't get collected
by the detector.

274
00:15:53,850 --> 00:15:56,070
MARK HARTMAN: It
didn't get detected.

275
00:15:56,070 --> 00:15:59,110
The photons hit there, but
it didn't get detected.

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00:15:59,110 --> 00:16:00,135
What about green light?

277
00:16:00,135 --> 00:16:01,510
What happened to
the green light?

278
00:16:01,510 --> 00:16:06,090
Here we had 16, and
here we have 10.

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00:16:10,850 --> 00:16:12,280
AUDIENCE: It reduced even more.

280
00:16:12,280 --> 00:16:13,363
MARK HARTMAN: It did what?

281
00:16:13,363 --> 00:16:14,910
AUDIENCE: It reduced even more.

282
00:16:14,910 --> 00:16:16,950
MARK HARTMAN: It
reduced even more

283
00:16:16,950 --> 00:16:20,830
because the detector response
is even lower at blue--

284
00:16:20,830 --> 00:16:23,580
or, I'm sorry-- at green light.

285
00:16:23,580 --> 00:16:25,780
What about blue?

286
00:16:25,780 --> 00:16:28,290
We started with 20 in blue.

287
00:16:28,290 --> 00:16:32,580
After the cloud, we
only had 12 left,

288
00:16:32,580 --> 00:16:34,935
and down here the detector
only picks up four.

289
00:16:38,490 --> 00:16:39,969
So what happened?

290
00:16:39,969 --> 00:16:41,760
AUDIENCE: It's the
least sensitive to blue.

291
00:16:41,760 --> 00:16:43,920
MARK HARTMAN: It's
least sensitive to blue,

292
00:16:43,920 --> 00:16:46,560
and the cloud knocked out
a lot of our blue light,

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00:16:46,560 --> 00:16:50,490
just like we saw over
there in the bin.

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00:16:50,490 --> 00:16:53,790
So what I want you
guys to do now.

295
00:16:53,790 --> 00:16:55,340
I want--