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Let's say I've got some
type of a book over here.

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So there is my book.

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I'll write it down, book.

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Let me draw the entire book.

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So this is my entire
book, just like that.

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And then I have a
mirror over here.

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So this is a mirror.

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We're just looking at the
mirror from the side--

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I can do a better job
of a straight line

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than that-- so that is a mirror.

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Now, what I want to do
is pick, and I'm really

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picking an arbitrary
point on this book.

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And we know that
there's light that

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is coming on this book--
maybe I haven't told us

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about the light source--
but we're assuming

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that the book will reflect
the light diffusely,

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like most books do.

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It is not a shiny book.

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And so any point on this book,
and we could pick any arbitrary

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point on this book, is
going to be emitting light

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in a whole bunch of directions.

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So what I'm just going to
do is pick two light rays

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that are being emitted from
this point on the book.

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It'll be more than
two, but this will

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help us understand
what's going on here.

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So let me pick
one light ray that

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goes like this-- it's obviously
not curving like that-- so it

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goes like this, and
hits the mirror.

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Incident angle is equal
to reflected angle,

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so it'll reflect off at the
same angle just like that.

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And let me pick another point
that is coming out radially

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from that same, or another ray
that is coming out radially

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from that same point, so
it'll come out like that.

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It has a larger
incident angle, so it'll

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have a larger reflected
angle, just like that.

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And now let's think
about what an observer,

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someone who has their
eyeball right over here--

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so let's say someone has
their eyeball right over here.

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Let me draw their
eyebrows so that you

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get the general idea-- let's
think about what this person

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right over here
will actually see.

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And remember all of
reality is really

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just a big simulation
going on in our brains.

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Now, this person's eye is going
to see these two light rays

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diverging from
some common point.

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If you were to look
straight at the book-- let

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me just do another
example-- if someone

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were to just look at
any arbitrary object,

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let's say that's an
orange right over there.

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And just ignore all
the stuff on the left.

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If I were to look at an
orange, no mirror, nothing,

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no reflections,
or anything, if I

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were to look at a certain
point on the orange,

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light from that point would
be diverging out radially.

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And if my eye were
right over there,

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my brain says, OK, I
have these light rays

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diverging from a single point.

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It will simulate,
or it will show

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me that point of an
orange in my brain.

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Now, The same thing is
going to happen here,

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even though these
two rays actually

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aren't diverging from
a point out here.

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Your brain will say, OK, I got
two things that are coming out

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from what looks like
a point over here.

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It'll really just extrapolate
these lines backwards.

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So it'll take that
green ray and it'll

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say, OK, it must be coming
from a point out there.

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And it'll take that
magenta ray and it'll say,

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it must be coming from
a point out there.

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Your eye is just seeing
these two light rays,

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it doesn't know what's
going on over here.

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It just says OK, they're
just converging from a point.

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And so in this person's
reality, or in my reality,

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I think that there
is an orange there.

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Likewise, I see
these two light rays

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diverging from
some common point.

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I will extrapolate
to figure out--

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or my brain will try to think
about-- where that point is

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in three dimensional space.

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It says, look,
these two rays look

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like they're coming from
a point back over here.

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Now, the reality is, there
is no back over here.

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There is nothing behind the
mirror, but to this observer,

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it looks like the light is being
emitted behind the observer.

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And so what the person
would actually see

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is something like this.

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And this is probably
not too foreign to any

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of our experiences.

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We've all looked in
mirrors-- and remember,

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this is the left side of
this book-- oh, sorry.

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This is the right
side of the book.

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It's the side that has
the curvy part of the B

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so it'll look like this.

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So what this observer sees is
an image that looks like this.

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It is what people
call the mirror image.

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It is flipped.

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The left and the
right is flipped

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and you can think
about why that is.

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I could just draw another ray
right here to make it clear.

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If this ray is coming like this.

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Now, it will reflect and
come out back like this

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to this observer it came from
that point right over there.

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So he sees a flipped
version of the book.

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Now the whole reason why I did
this is one, just to make you

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feel a little comfortable
with these reflecting

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lines and all of that.

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But one, I guess
more importantly

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to get you more comfortable with
the idea of a virtual image.

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This right here
is a virtual image

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and we're going to compare
it with a real image

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in the next video.

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And to some degree,
a virtual image

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is more intuitive because we
have so much experience with it

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when we think about mirrors
or reflective surfaces.

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So it's a virtual image.

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And we call it a virtual
image because the book really

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isn't there.

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It's virtually there.

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There is no actual
physical space,

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or we don't know if
there's any physical space

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behind the image.

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Our brain just uses
these diverging lights

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and creates a model
inside of our brain

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to say, OK, that
book exists there,

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even though the source of
the light is over here.

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Now I'm just going to
draw another diagram,

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and you normally won't
have the diagrams that

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have this type of
perspective, but just

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to give you a sense
of the same thing.

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And just to get you familiar
with some of these diagrams

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that we're going to do when
we study mirrors and lenses,

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is we could imagine
that this is the ground,

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let's say that this is a mirror.

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And let's say that
this is actually

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could be somebody standing
in front of a mirror.

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Although usually people
will draw an arrow here

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to just say an arbitrary object,

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Well, let's just say we have an
arrow in front of the mirror,

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just so you get used
to this notation.

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This is actually a physical
arrow, this is not a light ray.

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Now let's think about what
the image of this arrow--

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let's say this arrow had eyes,
so it's actually a person.

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So this arrow had eyes,
what would this arrow see?

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Well, let's just pick an
arbitrary point of this arrow.

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You could do it with any
point-- this object--

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because every point will be
reflecting light diffusely.

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So I'm going to take
one point that's

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coming straight out of the
mirror-- one light ray that

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goes straight to the mirror--
so just parallel to the ground,

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straight to the mirror.

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Well, it's just going to reflect
straight off and then come back

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to the source of that light.

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Maybe that arrow's eye, if
you view it as a person.

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And let's just do another
point, another ray

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coming from that point.

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So another ray coming from that
point might come out like that.

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Incident angle is equal
to reflected angle,

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so then it would
come back like that.

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Now, maybe this whole
thing is an eye.

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Maybe that'll simplify things.

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So let's say this
whole thing is you're

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looking really
close at a mirror.

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This whole thing is an eye.

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Now what would the eye see?

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Well, it's looking--
this ray looks

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like it's coming
from back over here.

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And then this ray
looks like it's

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coming from back over here.

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This is where they
converge, so the eye

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would see this point
of the eye over there.

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So it will actually see itself.

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And in general, these
type of diagrams

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are used to think about what
would the real or virtual image

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be of an object once the
light from that object

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either is reflected
off of a mirror,

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or goes through
some type of lens.

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And we'll be doing this a
lot more in future videos.

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But anyway, hopefully that
gets you a little bit familiar

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with the things we'll
be dealing with.