- [Dianna] Thanks to Curiosity Stream for supporting PBS.
So look through the straw at the coin
and then move the straw up and down.
- [William] Oh, wait, what?
What? This is weird.
Wait a second. What the heck is going on?
- Hey, I'm Dianna, and you're watching "Physics Girl."
I found out recently that straws are, like,
really entertaining.
You can create a mini explosion with a straw.
(pop)
Make a coin appear to disappear
by filling a glass with water.
That one has nothing to do with straws.
This video is about what happens
when you look through a straw at something underneath.
One of you sent me a tweet that sent me off
on a distracted tangent from life.
@theplanetmike asked me to help him explain
what you see when you look through a drinking straw.
He claimed that you see these weird rings,
but as I was fidgeting,
trying to figure out what he was talking about,
I noticed something really weird.
Oh, what? That's crazy.
(laughs)
I found a mystery, an everyday mystery.
- There's something that I am very interested to know.
- This is serious business.
Okay, listen closely.
If you put a penny underneath a glass of water
or a piece of paper with some writing on it,
and then you look down through a straw through the water,
being careful not to poke your eye out,
if you move the straw downwards,
you'll notice whatever you're looking at gets really big,
really magnified.
And then if you move the straw back up, it gets tiny.
And then as the end of the straw gets closer to the surface,
it gets really tiny.
Oh, that is so crazy.
Let me clarify: this isn't just because
you're moving the straw further away from the penny,
though it would be amusing if I had just discovered
depth perception for the first time,
but this effect works even when you're moving the straw
just a tiny bit back and forth.
And with other fluids,
it doesn't work, or it works differently.
Like in alcohol, you can see there's barely any effect
as compared to water.
We were moving the straw here
even more in the alcohol than we were in the water
when we took this footage.
We also tried this with a giant straw, a.k.a., a pipe,
and found the effect wasn't as pronounced.
And my office mates probably think I'm nuts.
I spent way too long playing with this.
(laughs)
That is...
(laughs)
Levi, you gotta try this. Let me tell you.
- [William] What's happening?
- [Dianna] Thought you'd never ask.
I think...
Well, I'm pretty sure,
when I push the straw down,
the water sticks to the inside walls of the straw,
so the straw is pulling the water down on the sides,
leaving a bulge in the middle,
and that forms a convex shape on the surface of the water,
like a magnifying lens.
And so it magnifies the image that you're looking at
at the bottom, like this bulgy glass lens magnifies things.
On the other hand, pulling the straw up
drags the water up on the sides,
leaving a convex dip in the water surface,
which makes the image appear much smaller,
like this glass lens does.
This lens is whatever the opposite of a bulgy lens is.
Saggy lens?
Yeah, that's good.
So you're creating a real lens. It's just temporary.
This all makes me wanna know why is it
that water can form these curve shapes on its surface
in the first place?
It all comes from the fact that water sticks to itself.
I keep wanting to use the word "cohesive,"
but it's very jargon-y, and essentially it just means
that molecules of the same material
stick to each other really well, like water,
because water has these strong hydrogen bonds.
So we'd say that water is very cohesive.
This leads to surface tension.
Okay, so imagine you've got water molecules
in the middle of the water.
They're surrounded by other water molecules.
They're all sticking to each other,
but at the surface of the water,
you've got this layer of water molecules
that don't have any molecules above them.
So they're only being pulled down
by all the molecules underneath,
which results in this very strong sort of mesh
of water molecules at the surface.
Because of its cohesion, the surface of water can support
these bulging shapes, like a meniscus or a lens.
And the reasons the effect didn't work as well with alcohol
is both because it doesn't have
as much surface tension as water
and because it probably sticks less
to the sides of the straw.
It has less adhesion, another word.
And these surface effects work best at small scales,
which is why it works better with the straw
and not the giant tube.
- [William] Wait, so you're creating a lens
inside of the straw?
- [Dianna] Yeah. But then-
So, water lenses.
That's what's going on.
By the way, water lenses are not new.
Your eyeball has a lens that's made up of mostly water.
I made another video with a half ring vortex in the pool,
and the dips created in the water formed a lens
that bent light away from a central point
and made these dark circles.
And also, you could hypothetically make a semipermanent lens
in microgravity, which is exactly what this astronaut,
Don Pettit, did in the space station.
Let me know if you ever end up trying this at home.
Tweet me.
And if you like straw-based experiments,
I did another, older video, more embarrassing video,
about getting a ball to levitate using just a straw.
Mystery solved.
Another case in the bag.
Thanks for watching, you guys, and happy physics-ing.
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(cheerful music)