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Before doing more examples with
Snell's Law, which essentially

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amount to some math
problems, what I want to do

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is get an intuitive
understanding

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for why this straw looks bent
in this picture right over here.

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To do that, let me
just draw a simplified

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version of that picture.

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So let's draw, this is
the cup right over here.

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We'll do a side
profile of the cup.

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So this is a side profile of
this glass right over here.

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It's the best that
I can draw it.

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And then let me draw
the actual straw.

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So I'm going to first draw the
straw where it actually is.

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So it's coming in off
the side of the cup,

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and the straw is
actually not bending,

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and it goes to the bottom
of the cup just like that.

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And then it goes up
like that, and then it

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goes slightly above it.

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And then it actually does
bend up here, up here, right

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over here, and then the
straw actually does bend.

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But that is irrelevant to
what we want to talk about.

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What I want to do in this video
is talk about why-- when we

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look over here, why does it
look like the straw got bent?

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And it all comes out
of the refraction

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of the light as the light from
the straw down here changes,

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as it goes from one
medium to another.

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Now, we know from
refraction indices,

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or just in general, that
light moves slower in water

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than it does in air.

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So it's slower in water
and faster in air.

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So let's think about
what's going to happen.

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Let me draw two
rays that are coming

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from this point on the
straw right over here.

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So if I draw one
ray right over here,

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so I'm just going to pick
an arbitrary direction.

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So if I pick one
ray just like that.

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Now, when it goes from the
slower medium to the faster

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medium, what's going
to happen to it?

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And it's at a slight angle here,
so the left side of the ray

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is going to end up in the
air before the right side.

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And I'm just using this
as a way of the-- I'm

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using the car example
to kind of think

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about which way this
light's going to bend.

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So if you visualize it
as a car-- or sometimes

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people visualize as
a marching band--

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the left side of
the marching band

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is going to get out
before the right side.

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And it's going to
start moving faster.

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So this is going to
turn to the right.

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Now, let me do another ray.

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So let me do another ray that's
going from that same point.

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I don't want it to go
right along the straw,

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so another ray just like that.

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It will also turn to the right.

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So it is also going
to turn to the right.

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Now, if someone's eye is right
over here, so that's your eye.

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That's the eyelashes.

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That someone's eye.

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You can draw their
nose and all the rest.

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If they're looking
down, where does it

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look like these two
light rays-- let's

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say their eye is big enough that
it captured both of these rays.

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Where does it look like these
two rays are coming from?

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So if you trace
both of these rays

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back, if you just assume
that there was a line here,

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that's what our eyes
and our brains do.

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If you assume that whatever
direction this ray is currently

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going is the direction
it came from,

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and same thing for this
magenta ray, just like that,

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it would look to this observer
that this point on the straw

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is actually right over there.

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And it would look--
and if you kept

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doing that for a bunch
of points on the straw,

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it would look like
this point on the straw

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is actually right over here.

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It would look like we could do
it for this point on the straw.

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It would look like
that point on the straw

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is actually right over here.

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So to this observer, the
straw would look like this.

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It would look like
something like that.

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It would look bent.

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This part would-- even
though the light from here

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is going up and then up
and then it moves out,

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because it gets bent,
when you converge it back,

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it would converge to
this, just like we

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saw with that first point.

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The light from this point,
when it goes out and gets bent,

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if you were to just
extrapolate backwards

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from their new directions,
you'd get to that point.

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So to this observer,
this point on the straw

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will look to be right over
here, even though the light was

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emitted down here.

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And that's why the straw
actually looks bent.

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So this is all really just
because of refraction,

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from going from a slow or
medium to a faster one.

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So hopefully you find that
a little bit interesting.

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In the next video, we'll
actually do some examples

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with Snell's Law
just to get ourselves

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comfortable with
the mathematics.

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