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MARK HARTMAN: I'm going
to introduce the particle

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model of light, or what we're
going to call the particle

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00:00:24,305 --> 00:00:24,930
model of light.

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So our CAI particle--
and again, you

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want to write down today's date.

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Write down the time.

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We're going to introduce
this idea, CAI particle

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model of light.

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It's going to be based off
of all of our observations.

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I'm going to say that--

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and again, this is
a model, so it's

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going to explain
why things happen.

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What do I say here?

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Light is composed of
particles of energy

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that move away from a source--

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and each one of
these words is going

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to have a specific meaning.

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So source, meaning something
that gives off light--

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in straight lines
in all directions--

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and I'm also going to
add this other thing--

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at the same speed.

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So this is what
we think, or a way

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to think about what light is--

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particles of energy.

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So we're just going to think--

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because we saw that, when you
turned a light on and pointed

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it towards a glass
of water, that it

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would heat up the water.

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So it has something
to do with energy.

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We're going to say
particles of energy

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because we also saw that
those particles move away

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from a source in
straight lines, just

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like when we had
the laser pointers.

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We also saw that they
went in all directions.

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When we were looking
at those shadows,

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light goes out not just in
one direction from a source.

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I mean, a laser does
because it's focused down,

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but in this case,
for our light bulbs,

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it goes out in all directions.

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We're also going to think
it's particles of energy

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because we said that light
can bounce or reflect.

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If you think about
taking a tennis ball

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and throwing it against a
wall, that tennis ball is going

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to bounce off in a
different direction,

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or if you take one of
those super bouncy balls

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and you throw it down on the
ground, it bounces up and down.

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So we're going to think that
these particles are bouncy.

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Now, particles of energy--

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what does that really mean?

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Well, we're not going to get
into the philosophical deep

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understandings of light just
because, like I said before,

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even this model doesn't
explain everything.

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It's going to explain as
much as we need it to.

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So here is a drawing.

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I want you to put this
drawing in, as well.

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So if we had a source of light--

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here's a light bulb--

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we are going to draw--

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I'm just going to say
from this light bulb,

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from the source we are going
to get particles of light

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that we're going
to call photons.

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Each one of these
particles of energy--

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we're going to say
called photons.

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You can put that in, or you
could say down below particles

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of energy are called photons.

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We're going to say from
this source these photons--

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we're going to represent
the speed of these photons

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with an arrow, and it's
going to be pointing outward.

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And you can see that all my
arrows have the same length,

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which means that these
particles are going

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outward at the same speed.

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Now, it's not that there's
only one set of particles

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that gets thrown out.

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When you turn a light on, more
particles of energy come out.

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As we saw with the
light bulb, if you

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leave the bulb on all day,
more, and more, and more energy

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00:04:43,510 --> 00:04:47,140
gets transferred to the
water, and it heats up.

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So it's not just one
round of particles.

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There's also particles
out here, and they're

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going outward, too, still
in straight lines, still

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00:05:03,530 --> 00:05:06,831
all at the same speed.

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00:05:06,831 --> 00:05:09,080
Now, this is obviously only
a two dimensional drawing,

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so there's particles
that are coming

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out this direction, as well,
as well as back into the board.

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So here we can kind of see,
well, if we have our detector,

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and we put it up close
we're going to collect--

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00:05:24,530 --> 00:05:26,780
those particles
haven't spread out.

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It's not that the particles
themselves get bigger,

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but if these particles are
going in straight lines,

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00:05:32,550 --> 00:05:34,190
they start out close
together, and then

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00:05:34,190 --> 00:05:38,850
they get further out as you
go away from the source.

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00:05:38,850 --> 00:05:40,940
So if you're far
away, you're not

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going to collect as many
particles with the same area

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as if you were up close.

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We're going to see
that in just a minute.

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AUDIENCE: So you can make
a box that is completely

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just solar panels and then
put the light bulb in the box

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and then turn it on so it
captures all the light that it

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hits, and then connect it
to one of those [INAUDIBLE]..

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MARK HARTMAN: Somebody
else want to explain

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that idea in their own words.

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That makes sense.

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00:06:09,660 --> 00:06:12,720
Juan, why don't you try?

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AUDIENCE: So they can
[INAUDIBLE] solar panels

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[INAUDIBLE] and they calculate
how much light you get out

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00:06:20,916 --> 00:06:21,552
of that--

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00:06:21,552 --> 00:06:24,110
I mean, how much energy.

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MARK HARTMAN: So if we could
put solar panels everywhere

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around this bulb, we could
collect all of the light.

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AUDIENCE: When I was
[INAUDIBLE] have a spot where

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there's all curved
mirrors all around it

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and focus in [INAUDIBLE].

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MARK HARTMAN: So like this.

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00:06:49,720 --> 00:06:52,000
So why do people use
these reflectors?

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Why does everybody's car
have one of these-- two

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00:06:54,940 --> 00:06:56,228
of these in the front?

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00:06:56,228 --> 00:06:57,572
AUDIENCE: Focus [INAUDIBLE].

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AUDIENCE: To
concentrate the light.

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00:06:59,297 --> 00:07:00,130
MARK HARTMAN: Right.

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00:07:00,130 --> 00:07:03,400
If you just had a light bulb
out at the front of your car,

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00:07:03,400 --> 00:07:04,970
the light going this way--

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there's going to be some light
going this way, some going

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this way, some going backwards.

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You're losing that light.

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If Azeith, like you say,
if we took our light bulb

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and we put it in here, the
light that goes backwards gets

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reflected back out.

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So if we put this right here
and we had a detector over here,

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we could get maybe half
the light, and if we

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knew that there was half--

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go ahead and put it.

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Is it on?

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00:07:29,200 --> 00:07:31,280
What are you bringing
this up here for?

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AUDIENCE: So for example,
this is a light bulb.

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[INAUDIBLE]

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MARK HARTMAN: Turn.

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AUDIENCE: Where
did he come from?

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00:07:38,370 --> 00:07:39,358
He was over there.

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MARK HARTMAN: I think
the battery has died.

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AUDIENCE: My bad.

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MARK HARTMAN: Do you have
another one of these?

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What are we going to do with it?

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AUDIENCE: So my
explanation was going to be

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this is the light bulb, and--

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MARK HARTMAN: Sorry.

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AUDIENCE: It's all right.

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So how you guys see inside
this [INAUDIBLE] is a curve,

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and it's shiny like a mirror.

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00:08:00,780 --> 00:08:07,360
So you can collect those
photons to focus the light

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and to reflect the light.

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MARK HARTMAN: There we go.

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So show us with that one.

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AUDIENCE: Watch [INAUDIBLE].

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AUDIENCE: Anyways,
see it's like--

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00:08:20,200 --> 00:08:25,320
can you see all this
light is coming straight.

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And if you put this
thing, it's going

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to be focusing in one spot.

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MARK HARTMAN: So if we have
some kind of reflector,

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if we knew what
fraction of the total

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00:08:39,820 --> 00:08:41,559
we collected-- if
we put this here

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and we collected half
the light over here,

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then we could just take
the amount of light

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that we collected and multiply
by 2, and we'd know the total.

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00:08:49,400 --> 00:08:52,900
So what I want to
do is let's assume--

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00:08:52,900 --> 00:08:57,520
let's try and recreate
Bianca and Juan's idea.

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Here is our star right here.

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I want everybody to
stand up, and each of you

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is going to get a detector.

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00:09:07,630 --> 00:09:10,390
Remember, we said detectors
are these pieces of silicone

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00:09:10,390 --> 00:09:13,720
or some other material that
have a bunch of squares,

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00:09:13,720 --> 00:09:15,950
pixels on them.

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00:09:15,950 --> 00:09:19,780
And if I were
standing right here,

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00:09:19,780 --> 00:09:26,380
I could detect all the light
that came into my detector,

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because back here
there's a shadow.

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I'm stopping all the light.

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The light gets absorbed there.

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So what I want to do
is I want everybody

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00:09:33,340 --> 00:09:36,190
to have detector,
and let's collect

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00:09:36,190 --> 00:09:38,089
all the light from this object.

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00:09:38,089 --> 00:09:39,880
And can we turn the
lights back down again?

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00:09:43,660 --> 00:09:49,030
But let's collect all the
light about this far away.

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00:09:49,030 --> 00:09:52,485
So let's collect all the light.

190
00:09:52,485 --> 00:09:55,810
So I'm going to stand
here next to you, Azeith.

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00:09:55,810 --> 00:09:58,999
So we're missing some
light out the top.

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AUDIENCE: [INAUDIBLE].

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MARK HARTMAN: Here's
another couple.

194
00:10:01,450 --> 00:10:04,027
Everybody has two hands.

195
00:10:04,027 --> 00:10:05,860
You don't want to stick
it in to the middle.

196
00:10:05,860 --> 00:10:09,400
You want to--
let's all kind of--

197
00:10:09,400 --> 00:10:10,330
here we go.

198
00:10:10,330 --> 00:10:15,010
No, you don't-- so where
are we missing light?

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00:10:15,010 --> 00:10:15,830
AUDIENCE: Here.

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00:10:15,830 --> 00:10:17,220
[INTERPOSING VOICES]

201
00:10:17,220 --> 00:10:18,610
MARK HARTMAN: At the bottom.

202
00:10:18,610 --> 00:10:19,610
So I'll add another one.

203
00:10:19,610 --> 00:10:21,360
Azeith, here, why don't
you take that one.

204
00:10:21,360 --> 00:10:23,130
Put it down there.

205
00:10:23,130 --> 00:10:28,330
So I want you to look at
how intense your detector is

206
00:10:28,330 --> 00:10:29,290
lit up.

207
00:10:29,290 --> 00:10:31,570
And Jaylen, you're
not getting anything,

208
00:10:31,570 --> 00:10:34,731
so why don't you come
down here on the bottom.

209
00:10:34,731 --> 00:10:35,230
There we go.

210
00:10:35,230 --> 00:10:36,390
Now we've covered it up.

211
00:10:36,390 --> 00:10:38,140
AUDIENCE: I'm covering
most of the bottom.

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00:10:38,140 --> 00:10:43,640
MARK HARTMAN: Except-- so
Juan, you compare how bright--

213
00:10:43,640 --> 00:10:44,560
well, OK, fine.

214
00:10:44,560 --> 00:10:46,500
So everybody sees that.

215
00:10:46,500 --> 00:10:47,000
There we go.

216
00:10:47,000 --> 00:10:49,780
We've collected all of them.

217
00:10:49,780 --> 00:10:51,400
Close enough.

218
00:10:51,400 --> 00:10:54,250
Now I want everybody to
take a step backwards.

219
00:10:54,250 --> 00:10:55,044
AUDIENCE: Wow.

220
00:10:55,044 --> 00:10:57,460
MARK HARTMAN: What just happened
to the intensity of light

221
00:10:57,460 --> 00:10:59,043
that you collected
with your detector?

222
00:10:59,043 --> 00:11:00,200
AUDIENCE: It decreased.

223
00:11:00,200 --> 00:11:01,033
MARK HARTMAN: Right.

224
00:11:01,033 --> 00:11:02,726
If you put it close--

225
00:11:02,726 --> 00:11:03,604
AUDIENCE: Increase.

226
00:11:03,604 --> 00:11:05,770
MARK HARTMAN: --that's a
lot that you're collecting.

227
00:11:05,770 --> 00:11:06,749
If you put it back--

228
00:11:06,749 --> 00:11:07,540
AUDIENCE: Decrease.

229
00:11:07,540 --> 00:11:08,500
MARK HARTMAN: --you're
collecting fewer--

230
00:11:08,500 --> 00:11:09,980
there's fewer of
those particles,

231
00:11:09,980 --> 00:11:13,330
because they spread out
as they get out here.

232
00:11:13,330 --> 00:11:15,304
Now let's try and
collect all the light.

233
00:11:15,304 --> 00:11:16,220
What do we need to do?

234
00:11:16,220 --> 00:11:17,920
AUDIENCE: We need
more detectors.

235
00:11:17,920 --> 00:11:19,120
MARK HARTMAN: We
need more detectors.

236
00:11:19,120 --> 00:11:19,990
AUDIENCE: Or just go closer.

237
00:11:19,990 --> 00:11:20,980
AUDIENCE: [INAUDIBLE].

238
00:11:20,980 --> 00:11:22,300
MARK HARTMAN: Or go closer.

239
00:11:22,300 --> 00:11:25,120
Do you think you could go fly
to a star and get really close?

240
00:11:25,120 --> 00:11:26,320
As close as you want to?

241
00:11:26,320 --> 00:11:27,459
AUDIENCE: It's possible.

242
00:11:27,459 --> 00:11:28,000
AUDIENCE: No.

243
00:11:28,000 --> 00:11:29,125
AUDIENCE: It'd be fun.

244
00:11:29,125 --> 00:11:30,250
MARK HARTMAN: Probably not.

245
00:11:30,250 --> 00:11:32,250
AUDIENCE: Well, we don't
have as many detectors.

246
00:11:32,250 --> 00:11:34,360
MARK HARTMAN: We don't
have as many detectors.

247
00:11:34,360 --> 00:11:37,630
So if we're further away, we're
going to need more detectors

248
00:11:37,630 --> 00:11:40,270
to collect all the
light, and each detector

249
00:11:40,270 --> 00:11:43,880
is going to collect less,
because if you look at this,

250
00:11:43,880 --> 00:11:45,140
it's illuminated less.

251
00:11:45,140 --> 00:11:48,140
If you put it close,
it's illuminated more.

252
00:11:48,140 --> 00:11:50,330
The intensity is higher.

253
00:11:50,330 --> 00:11:53,860
So what I want you
to do with a group--

254
00:11:53,860 --> 00:11:55,060
well, yeah.

255
00:11:55,060 --> 00:11:56,630
We're running out of time here.

256
00:11:56,630 --> 00:11:58,720
So if we are--

257
00:11:58,720 --> 00:12:03,790
say we are all on
different planets,

258
00:12:03,790 --> 00:12:08,660
and we are all this
far from another star.

259
00:12:08,660 --> 00:12:11,980
What is the only thing
that I can measure here?

260
00:12:11,980 --> 00:12:15,051
Can I measure what's
in Peter's detector?

261
00:12:15,051 --> 00:12:15,592
AUDIENCE: No.

262
00:12:15,592 --> 00:12:16,420
AUDIENCE: No.

263
00:12:16,420 --> 00:12:17,128
MARK HARTMAN: No.

264
00:12:17,128 --> 00:12:18,880
I can measure what's
in my detector.

265
00:12:18,880 --> 00:12:22,120
How could I take
what I measure here

266
00:12:22,120 --> 00:12:26,490
and predict how much light
that source is putting off

267
00:12:26,490 --> 00:12:27,540
altogether?

268
00:12:27,540 --> 00:12:29,280
What would I need to do?

269
00:12:29,280 --> 00:12:31,294
AUDIENCE: Collect [INAUDIBLE].

270
00:12:31,294 --> 00:12:32,502
MARK HARTMAN: Say that again.

271
00:12:32,502 --> 00:12:34,543
AUDIENCE: Multiply it by
the number of detectors.

272
00:12:34,543 --> 00:12:37,360
MARK HARTMAN: So if I know
that this detector is this big,

273
00:12:37,360 --> 00:12:39,850
and if I knew how many
other detectors I would need

274
00:12:39,850 --> 00:12:43,750
to cover the whole star,
I could take my amount

275
00:12:43,750 --> 00:12:46,757
and just multiply
by that number.

276
00:12:46,757 --> 00:12:49,090
In other words-- and this is
what we're going to go back

277
00:12:49,090 --> 00:12:50,080
and write down--

278
00:12:50,080 --> 00:12:53,650
when you collect
from your detector,

279
00:12:53,650 --> 00:12:57,910
you only collect a fraction
of the total amount of light.

280
00:12:57,910 --> 00:13:01,990
That fraction is the
area of your detector

281
00:13:01,990 --> 00:13:04,780
divided by the area of what?

282
00:13:04,780 --> 00:13:06,820
What do we need to make
here with detectors?

283
00:13:06,820 --> 00:13:08,327
AUDIENCE: Sphere.

284
00:13:08,327 --> 00:13:09,660
MARK HARTMAN: We need to make a?

285
00:13:09,660 --> 00:13:10,560
AUDIENCE: Sphere.

286
00:13:10,560 --> 00:13:12,185
MARK HARTMAN: We need
to make a sphere,

287
00:13:12,185 --> 00:13:13,580
because if we went up close--

288
00:13:13,580 --> 00:13:17,702
everybody go up close and make
a sphere with your detectors.

289
00:13:17,702 --> 00:13:19,535
AUDIENCE: I'm going to
have to [INAUDIBLE]..

290
00:13:19,535 --> 00:13:21,310
MARK HARTMAN: Everybody should
be about the same distance

291
00:13:21,310 --> 00:13:21,809
away.

292
00:13:21,809 --> 00:13:24,220
So Azeith, you're too close.

293
00:13:24,220 --> 00:13:27,670
So if we bent this around, this
shape that we're omitting here

294
00:13:27,670 --> 00:13:30,830
is kind of a sphere shape.

295
00:13:30,830 --> 00:13:33,320
So if I knew the
area of my detector

296
00:13:33,320 --> 00:13:36,260
and I knew the area
of this sphere,

297
00:13:36,260 --> 00:13:41,070
I could figure out how much
light this object put out.

298
00:13:41,070 --> 00:13:42,720
So I want you to
keep that in mind.

299
00:13:42,720 --> 00:13:45,440
We're going to come back
to that a little bit.

300
00:13:45,440 --> 00:13:49,172
So what do I need to know to
figure the area of that sphere?

301
00:13:49,172 --> 00:13:50,450
AUDIENCE: Area?

302
00:13:50,450 --> 00:13:52,940
MARK HARTMAN: Yeah, the
area of this sphere.

303
00:13:52,940 --> 00:13:54,710
Does anybody
remember the formula

304
00:13:54,710 --> 00:13:57,830
for the area of a sphere
from geometry class?

305
00:13:57,830 --> 00:13:59,790
AUDIENCE: 4 pi r squared.

306
00:13:59,790 --> 00:14:02,750
MARK HARTMAN: So
the area of a sphere

307
00:14:02,750 --> 00:14:07,790
is 4 times pi times the
radius of the sphere squared.

308
00:14:07,790 --> 00:14:10,960
What is the radius of
our sphere in this case?

309
00:14:10,960 --> 00:14:12,320
Let's all make the sphere again.

310
00:14:15,247 --> 00:14:16,580
What's the radius of the sphere?

311
00:14:16,580 --> 00:14:18,957
Somebody point out the
radius of our sphere.

312
00:14:18,957 --> 00:14:19,540
AUDIENCE: 5.5.

313
00:14:19,540 --> 00:14:22,370
AUDIENCE: The
light bulb to the--

314
00:14:22,370 --> 00:14:25,980
MARK HARTMAN: The light
bulb to the detector.

315
00:14:25,980 --> 00:14:27,890
So if I'm here by
myself, I would just

316
00:14:27,890 --> 00:14:31,824
say, well, that's the distance
from my detector to the source.

317
00:14:31,824 --> 00:14:32,990
AUDIENCE: But in actuality--

318
00:14:32,990 --> 00:14:35,780
MARK HARTMAN: But in actuality,
it's the radius of this circle

319
00:14:35,780 --> 00:14:37,895
if I were to collect
all of the light.

320
00:14:40,820 --> 00:14:44,450
So let's turn this off so
we don't get too blind.

321
00:14:44,450 --> 00:14:46,160
Let's turn the lights
back on, and I just

322
00:14:46,160 --> 00:14:48,080
want to write that down, and
I want you guys to write that

323
00:14:48,080 --> 00:14:48,980
down in your notes.

324
00:15:01,540 --> 00:15:19,920
We're going to say,
how can we collect

325
00:15:19,920 --> 00:15:22,320
all the light from an object?

326
00:15:22,320 --> 00:15:23,837
That's what we're wondering.

327
00:15:28,980 --> 00:15:31,900
There are two ways.

328
00:15:31,900 --> 00:15:46,300
We could either surround
the source with detectors,

329
00:15:46,300 --> 00:16:01,335
or we could collect
light with one detector.

330
00:16:06,860 --> 00:16:11,670
If we collect the light
with one detector,

331
00:16:11,670 --> 00:16:29,100
then we want to say the fraction
of the total light emitted

332
00:16:29,100 --> 00:16:30,307
is equal to--

333
00:16:30,307 --> 00:16:31,890
actually let me put
this on two lines.

334
00:16:41,750 --> 00:16:43,340
Fraction of the
total light emitted

335
00:16:43,340 --> 00:16:47,930
is equal to the area
of my collector--

336
00:16:53,150 --> 00:16:55,820
well, actually I said collect
light with one detector.

337
00:16:55,820 --> 00:16:57,270
Let's say-- yeah.

338
00:17:04,160 --> 00:17:06,410
Now and I'm going to change
from detector to collector

339
00:17:06,410 --> 00:17:10,069
because it's not that we just
have that one little chip,

340
00:17:10,069 --> 00:17:13,010
but we actually have the
whole telescope, the aperture

341
00:17:13,010 --> 00:17:15,710
of the telescope that
then collects the light,

342
00:17:15,710 --> 00:17:17,040
and it focuses the light.

343
00:17:17,040 --> 00:17:21,286
Remember, when we looked at
making an image with a mirror,

344
00:17:21,286 --> 00:17:23,119
we were collecting the
total amount of light

345
00:17:23,119 --> 00:17:27,919
that hit the mirror, not just
the light on the detector.

346
00:17:27,919 --> 00:17:29,960
So we could collect the
light with one collector.

347
00:17:29,960 --> 00:17:31,585
The fraction of the
total light emitted

348
00:17:31,585 --> 00:17:39,290
is the area of the collector
over the area of a sphere.

349
00:17:46,890 --> 00:17:52,590
We're just going to say
the area of a sphere

350
00:17:52,590 --> 00:18:08,332
with radius equal to your
distance from the object,

351
00:18:08,332 --> 00:18:10,790
because whenever we're working
with something in astronomy,

352
00:18:10,790 --> 00:18:12,440
we can never collect
all the light.

353
00:18:12,440 --> 00:18:14,540
We can't go out in space,
and we can't do this.

354
00:18:14,540 --> 00:18:17,780
We can't surround every star
with a bunch of detectors.

355
00:18:17,780 --> 00:18:19,880
What we can do is
we can collect light

356
00:18:19,880 --> 00:18:23,060
with one collector
or one telescope,

357
00:18:23,060 --> 00:18:25,930
and so long as we know that--

358
00:18:25,930 --> 00:18:27,680
so long as we can
figure out what fraction

359
00:18:27,680 --> 00:18:30,300
of the total light,
then we can figure out

360
00:18:30,300 --> 00:18:32,300
how much light the overall
thing is putting out.

361
00:18:40,240 --> 00:18:43,290
So I'm going to draw a little
diagram to go along with this.

362
00:18:43,290 --> 00:18:53,090
If you have a light bulb here,
if you are this far away,

363
00:18:53,090 --> 00:18:58,460
this is your collector,
and normally we

364
00:18:58,460 --> 00:18:59,935
say that's a telescope.

365
00:19:05,050 --> 00:19:12,115
This is your distance
to the object.

366
00:19:16,440 --> 00:19:19,220
So you need to know
your area of a collector

367
00:19:19,220 --> 00:19:27,320
compared to the area
of a sphere that

368
00:19:27,320 --> 00:19:30,260
is centered at that
object that has

369
00:19:30,260 --> 00:19:34,050
the same radius as the distance
between you and the object.

370
00:19:36,391 --> 00:19:38,140
So we're never going
to get all the light.

371
00:19:38,140 --> 00:19:40,860
We're just going to get
a fraction of the light.