[Focal Length of a Converging Lens]
Dan: What's this object here on the optics track here, JJ?
JJ: Well, Dan, it is a light source.
There's a lamp inside here.
There is a translucent screen here that's illuminated by the light source that's inside.
What we're going to do is we're going to use this in our experiment as our object.
Dan: It looks like something you might see at your optometrist.
JJ: Yeah, that's right.
There are lines on here to make measurements both left and right, and up and down,
for additional experiments if we wanted.
Dan: Here in the middle that looks like a convex lens,
and that's something else that is at the optometrist.
It's what he's studying in your eye, right?
JJ: Yeah, that's exactly right.
This is a double convex lens, or a converging lens,
that we're going to use in our experiment.
We actually don't know what the focal length is.
That's what this experiment is based on.
We're going to collect some data and try to experimentally determine
what the focal length of that lens is.
Dan: I think we need to measure the object distance and then the image distance to do that.
JJ: Right.
Let's start with an object distance of 24 centimeters.
That's the distance between the screen here and the lens.
This is our object distance.
I'm going to use the scale here on this track as my guide. 24 centimeters.
Now this lens should produce a real image.
Dan: But I don't see anything there.
JJ: OK, so let's adjust this.
We're going to adjust this viewing screen until we see the image in focus.
I'm going go back and forth until I find it.
That looks good right about there.
Dan: The image distance would be from the lens to the screen then.
JJ: Right.
Once it's in focus, this distance is the image distance.
This looks like 18.4.
Dan: Got that.
We should maybe move it apart another 10 centimeters or so.
JJ: Yeah, so we're going to collect some more data
to help us determine this relationship graphically.
Let's move this.
Let's make this now instead of 24, we'll do 34.
We'll add 10 centimeters.
I just move that 10 centimeters.
Moving it made the image go out of focus.
We've got to figure out what the new image distance is.
So I'll just move that.
That looks about the most focused.
That is 15 centimeters.
Dan: So it went down as that went up.
JJ: That's right.
Let's keep going 10 more centimeters.
We've got 44 centimeters, right?
Dan: Right.
JJ: Then I'll adjust this.
Right about there's the most focused--just double-check.
This is 13.6.
Dan: OK, should do a couple more here.
JJ: We'll do 10 more.
Then we'll move that.
It's hard to tell sometimes when the image gets really small,
but that looks like it's pretty much in focus.
What do you think?
Want to take a look?
Dan: That looks good.
JJ: OK, so this is 13.
Dan: It's still going down.
JJ: 13.
One more.
Dan: One more should be good.
JJ: We'll move that 10 more centimeters.
We'll adjust this.
That looks good right there.
12.4.
Dan: OK, that's five data points.
I've entered them into this Capstone file, which you have access to.
Your job is to do the analysis to look at the relationships between this data,
make a graph, and come up with a thin lens formula,
and apply that to some questions.