[Snell's Law]
JJ: Dan, what do we got going on here?
Dan: We're going to investigate Snell's Law.
We have a light source that's producing a
beam of light
that is going through this D-shaped lens.
We're going to compare the angle of the incoming
beam to the outgoing beam;
the relationship between them is called Snell's Law.
You can't see the beam because of the lights,
so let's bring the lights down.
Now you can see it.
It's going straight in and it comes straight
out, so not a whole lot going on there but
as we rotate it, at some point, you can see
they're going in different directions.
We're going to measure the angle between the
normal and the incoming beam
and the normal and the outgoing beam.
From that, we can derive Snell's Law, which
is also based on Fermat's principle
that the light will take the path of least time from
point A to point B.
JJ: What do you mean by least time?
It's traveling the same speed, right?
Dan: One model would say that the light does
slow down when it goes through the lens.
For the light to go from one end to the other
end, if it took a straight-line path,
that would take it longer than the path that it's
taking here
because it'd spend more time going through the slower medium than the faster one.
JJ: You want to go ahead and make some measurements here?
Dan: How about we start off at...
JJ: What are the two quantities that we're
going to be measuring again?
Dan: The angle of the incoming ray from the normal
and the angle of the outgoing ray from its normal.
This PASCO Basic Optics Ray Table helps us
do that.
I'm setting the initial angle to 10 and the
outgoing one,
you may have trouble seeing that on the video, so I'm going to put a pencil  by it,
looks like at about 7 degrees to me, but let's bring the lights up so you can see.
JJ: Maybe just stand...
Dan: I'll get out of the way.
There we go.
You can see it's at about 7 degrees.
JJ: I'll enter that in our table.
The incident angle was 10 degrees, and the
refraction angle was 7 degrees.
Dan: Right.
Let's go up another 15.
The incident beam is at 25 degrees.
JJ: That's already in our table.
Dan: The outgoing one, if I put a pencil there,
looks like it's about 16 and a half, if you 
  want to go to halves.
JJ: 16.5
Dan: Now, another 15, so we're up to 40.
The outgoing looks like about 25 and a half.
JJ: 25.5 degrees.
Dan: Then another 15.
JJ: It's really starting to bend, now.
Dan: We're getting a little noticeable reflection
off the front surface, as well.
That could confuse you.
Now, we're at 55.
JJ: 55.
Dan: The outgoing beam is at 33 and a half.
JJ: 33.5.
Dan: Then one more.
JJ: 70.
Dan: 70.
You can definitely see that reflected beam,
now.
It's following the law of reflection.
This is the law of refraction. It looks to
be at 39.
JJ: 39.
OK, great.
Dan: They have all the data,
so they can do
the analysis to do a graph,
and come up with Snell's Law, and then use that to answer some of the questions in the lab.
The rest of the work is for you.
