Before doing more examples with
Snell's Law, which essentially amount to some math
problems, what I want to do is get an intuitive
understanding for why this straw looks bent
in this picture right over here. To do that, let me
just draw a simplified version of that picture. So let's draw, this is
the cup right over here. We'll do a side
profile of the cup. So this is a side profile of
this glass right over here. It's the best that
I can draw it. And then let me draw
the actual straw. So I'm going to first draw the
straw where it actually is. So it's coming in off
the side of the cup, and the straw is
actually not bending, and it goes to the bottom
of the cup just like that. And then it goes up
like that, and then it goes slightly above it. And then it actually does
bend up here, up here, right over here, and then the
straw actually does bend. But that is irrelevant to
what we want to talk about. What I want to do in this video
is talk about why-- when we look over here, why does it
look like the straw got bent? And it all comes out
of the refraction of the light as the light from
the straw down here changes, as it goes from one
medium to another. Now, we know from
refraction indices, or just in general, that
light moves slower in water than it does in air. So it's slower in water
and faster in air. So let's think about
what's going to happen. Let me draw two
rays that are coming from this point on the
straw right over here. So if I draw one
ray right over here, so I'm just going to pick
an arbitrary direction. So if I pick one
ray just like that. Now, when it goes from the
slower medium to the faster medium, what's going
to happen to it? And it's at a slight angle here,
so the left side of the ray is going to end up in the
air before the right side. And I'm just using this
as a way of the-- I'm using the car example
to kind of think about which way this
light's going to bend. So if you visualize it
as a car-- or sometimes people visualize as
a marching band-- the left side of
the marching band is going to get out
before the right side. And it's going to
start moving faster. So this is going to
turn to the right. Now, let me do another ray. So let me do another ray that's
going from that same point. I don't want it to go
right along the straw, so another ray just like that. It will also turn to the right. So it is also going
to turn to the right. Now, if someone's eye is right
over here, so that's your eye. That's the eyelashes. That someone's eye. You can draw their
nose and all the rest. If they're looking
down, where does it look like these two
light rays-- let's say their eye is big enough that
it captured both of these rays. Where does it look like these
two rays are coming from? So if you trace
both of these rays back, if you just assume
that there was a line here, that's what our eyes
and our brains do. If you assume that whatever
direction this ray is currently going is the direction
it came from, and same thing for this
magenta ray, just like that, it would look to this observer
that this point on the straw is actually right over there. And it would look--
and if you kept doing that for a bunch
of points on the straw, it would look like
this point on the straw is actually right over here. It would look like we could do
it for this point on the straw. It would look like
that point on the straw is actually right over here. So to this observer, the
straw would look like this. It would look like
something like that. It would look bent. This part would-- even
though the light from here is going up and then up
and then it moves out, because it gets bent,
when you converge it back, it would converge to
this, just like we saw with that first point. The light from this point,
when it goes out and gets bent, if you were to just
extrapolate backwards from their new directions,
you'd get to that point. So to this observer,
this point on the straw will look to be right over
here, even though the light was emitted down here. And that's why the straw
actually looks bent. So this is all really just
because of refraction, from going from a slow or
medium to a faster one. So hopefully you find that
a little bit interesting. In the next video, we'll
actually do some examples with Snell's Law
just to get ourselves comfortable with
the mathematics.