- So Derek, I have a riddle for you.
- All right.
Have you are roped into these things?
(Diana laughs)
- [Diana] Look at it.
- Yeah, I'm upside down. - [Diana] yeah.
- [Derek] Upside down right there?
- [Diana] Yeah. Flip it around.
- [Derek] And then I'm right side up.
- It's something that we all noticed
when we were like in second grade.
- [Diana] You know what? The riddle is...
Now, you could take this spoon
and you could push it through this way
until it's convex, facing you.
So what would happen to your image
as you're going from upside down to right side up?
- This is a tricky question.
- Hey, I'm Diana and you're watching Physics Girl.
If you take a spoon and you look at the scoop side,
the inside, then your reflection is upside down.
When you turn it the other way,
so the backside is facing you,
your reflection is right side up.
Have you ever seen this before?
Yeah? No? Cool.
Now how the flippy, image-y thing works
is already a little bit tricky,
but the setup to my riddle is a step further.
Imagine you could take this spoon,
and go from the concave side facing you, the scoop side,
to pushing it through until the convex side is facing you.
Got it?
So it's like pushing through one of those poppers.
So your image goes from upside down to right side up.
And at some point in between,
it's gonna be flat like a regular mirror.
And then it's curved toward you.
Obviously, at some point you, have to flip.
So the question is, how does your image flip?
Basically, how does your image flip
when you take a concave surface and warp it through
being flat all the way to a convex surface.
- It would like warp, like around.
- And you just get larger and larger and larger
as you approach the focal point.
- My image size will like blob to infinity
and essentially be kind of undefined.
- I don't think the image rotates.
- And as you're starting to find out the spoon,
it will continue to get larger,
but still be inverted up until the point
where you flatten that spoon.
- And then the moment it reaches flatness,
then it's back to the right side of normal.
- It'll go back down from infinity
and it will be right side up.
- All those video submissions of the answer
were from my Patrons who got the riddle early.
And apparently I have really smart Patrons
because they all had the right idea on a riddle
that I struggled with.
Okay, let's dig into this.
The answer has to do with how far away you are
from the spoon.
So in some ways it's a trick question,
because there's something that you can do
to get your image to flip
without even doing anything to this spoon.
This is pretty interesting to see.
If you take the spoon and move it closer and closer
and closer, really close to your eye,
you'll notice that your eyeball is right side up.
- Yeah, that's my eye.
- You can tell by blinking and noticing
that you blink normal direction.
- Is this actually for a YouTube video Diana?
- So that means that you've passed this point
where you're no longer upside down,
you're now right side up.
Which seems like a significant point to keep in mind
as we're going forward.
Okay, let's do a diagram.
With optics, it's necessary. It's so confusing.
Okay, here's the concave mirror and here's you.
if I draw a couple light rays from the top of your head
to the mirror and reflect them,
they'll eventually meet somewhere.
That's where the image is,
where the light rays actually meet,
where they "converge," as we say in physics.
That's also the definition of an image
and we've made a real, honest to goodness image of you.
As in, if you put a screen here, the light rays
would project a real visible image onto the screen.
When I turned the U haul truck into a camera Obscura,
that was a real image being projected on the wall
that you could see.
There's also such a thing as a fake image,
and we can see that happen if we move you
closer to the mirror, it's where the light rays
look like they're converging but they don't actually.
There's no light over here, it doesn't pass the mirror.
We call that a virtual image,
because if you put a screen there, nothing would appear.
There are no light rays there.
So actually, a regular flat mirror creates a virtual image
of your reflection, not a real image.
The only way that you can get a real image with a mirror
is if you have a curved concave mirror.
Okay, so let's go back and forth
between the real and virtual image in our scenario.
When does it switch from one to the other?
Right here.
Eventually, there comes a point where the light rays
are parallel to each other, so they're never gonna converge.
So you don't actually have any image there.
Okay, well, let's mark that point.
This happens to be the focal point of the mirror,
and this is a pretty pedantic point,
but I feel like I have to say this anyways.
This is only for strictly parabolic mirrors
or it's an approximation for spherical mirrors
that are not too curved.
Otherwise, they don't have one focal point,
they have aberrations that look like this
and it gets blurry and stuff, it's a mess.
Then, okay, keep moving, you move past the focal point
and the rays diverged.
But they appear to converge on the other side,
in a right side up virtual image.
This is exactly what you see if you take the spoon
and move it really close to you, eventually, the image flips
and you see it right side up.
If you wanna see it a little clearer,
take a look at one of those makeup mirrors,
like the ones that really magnify your face
and start far away from it, look at your image,
which should be upside down and then walk towards it,
eventually, it'll get bigger and bigger and bigger,
until it flips and you're right side up.
Derek from Veritasium happened to have this amazing,
perfectly parabolic mirror in his garage.
So we walked back and forth and looked at
what the image does. It's bizarre.
As you get closer and closer to the focal point,
your image gets massive and then it flips
as you move back and forth across the focal point.
So it seems like the image just suddenly flips
from upside down to right side up,
which seems like a discontinuity.
How is that happening?
Well, your eyes are actually deceiving you
because it's not one image flipping,
here are two separate images.
I know, I'm confused too.
But it was a real image,
and then it becomes a virtual image.
The same thing happens if we curve the mirror
instead of moving you.
Okay, so that's what happens to the image,
you go from a real image to a virtual image.
But what do you see? That's the question.
As you curve the mirror, the image gets bigger.
So that's part of the answer.
But then there comes a point where the image passes you
and then it's behind you,
but you're in front of the focal point.
So what happens there?
Well, there's no image you can see in front of you anymore.
So things are just blurry.
And then eventually, the virtual image
starts coming from the other side
and shrinks back down again.
So this is what you'd see with an animation.
Since it's not easy to bend a spoon,
we did an animation showing what would actually happen
if you look at a curved surface bending like this.
You notice the image gets bigger and blurry,
and then shrinks back down flipped.
One of my Patrons had this brilliant idea for a contraption
that takes a stretchy material and with a little pressure
from a bottle, she was able to bend the surface
inward and outward and see how it affects the image
of four colored lights shining above.
- And now your toy bopper spoon is convex.
- [Diana] So smart.
So all of this isn't just stuff of riddles.
It turns out bending mirrors and lenses
is actually really useful.
Optical telescopes use a sophisticated version of this demo
and what's called Adaptive Optics.
When you're imaging stars and galaxies from earth,
that starlight has to travel through Earth's atmosphere,
which distorts the light to make a blurry image.
Adaptive optics, essentially deploy deformable mirrors
that reverse the effect of atmospheric distortion.
And this makes it possible to create high quality images
without having to launch a telescope into space
beyond the atmosphere.
And then another application is in your eyeball.
The interocular lens is used when people with cataracts
or near-sightedness may choose to remove
their eyes natural lens and replace it
with this interocular lens,
which focuses light on the retina to form an image.
And some models of this type of lens are deformable,
meaning that they can allow the eye to focus
at different distances.
That's pretty cool.
I hope you enjoyed this riddle and if you've got any more
awesome physics riddles, send them to me on Twitter,
maybe @thephsyicsgirl or leave them in the comments.
I love riddles.
Thanks for watching and happy physics-ing.
Hey there!
One more thing I think that you'd like,
check out Antarctic Extremes,
a new mini series from PBS Digital Studios and NOVA
that journeys to the most remote laboratory on earth,
Antarctica.
In the videos you'll join hosts,
Caitlin Saks and Arlo Perez,
as they explore a world that sometimes hilarious,
sometimes gross, but always fascinating.
You can find Antarctic Extremes on PBS Digital Studios
new science channel, PBS Terra.
Check out the episode in the description below,
and tell them that Diana from Physics Girl sent you.