- Hey, I'm Diana and you're watching Physics Girl,
go look at a picture of a bubble
where I guess I could just show you one.
If you don't trust me 'cause we've never met.
Go search online for pictures of bubbles
and look at what colors are on the surface.
If you don't have colorblindness
for me, it's magenta yellow Green, sometimes,
sometimes a little bit of blue,
but never red.
In fact, if you search red bubble,
what do you think you get?
If you search it with two words,
then you get red bubbles,
but they're all clip art.
They're all fake.
An everyday mystery.
It doesn't seem like red bubbles are possible.
And I wanted to know if that's actually true.
And if so, why?
It turns out that the way bubbles get their colors.
I just I'm going to keep doing this
because there's oh, fun.
They're so cute too. Oh, no.
(chuckles)
Sorry.
Where was I?
So it turns out the, the way that bubbles get their colors.
Which come to think of it
is weird because they're made out
of a clear or like a, this is kind of a bluish mixture,
but whatever color your soap is,
so why do they even have a color at all?
But it's a phenomenon.
That's kind of the opposite of rainbows.
What I mean by that is that in bubbles colors are removed,
they are canceled.
And it all has to do with thin films,
which is something that you learn about
if you're taking physics and you get to
wave interference,
what's that?
Well, there's this phenomenon that happens sometimes
near a seashore.
It's really cool
where a wave comes in and crashes on the shore
and it can reflect back and travel backwards.
And then if it hits an incoming wave,
you get these monster waves. When they collide,
the monster wave happens because of something called
wave interference.
Or you can add up two waves
and they combine for a bigger wave
and it is pretty cool looking.
So now the same thing can happen with light
because light is a wave.
That's make sense
if you imagine shining two lights on a spot
they add, and they look even brighter
than if you have just one of the lights,
but you can also shine two lights
and get the spot to completely disappear.
You can get destructive interference.
So interference goes both ways.
If you have a crest and a crest to ups,
then you add and you get a bigger up,
I can't do it.
Or if you have a crest and a trough, an up and a down,
then they add and they cancel out and you get zero.
It can happen in ocean waves.
If you get like a, a crest and then the wave,
that's like a dip in the water and they add and,
they canceled to zero.
It can happen in sound waves,
which is how you get noise canceling headphones.
And it can happen in light waves
where the waves come together and cancel each other out.
This is a real thing with light,
you shine two lights and you get them to completely cancel.
They would probably have to be two laser lights
and of the same color
because of the particular properties of lasers,
but it's possible. And it's not even that hard.
And that's the basic idea of interference.
So the next step, this wave interference thing
that can make light cancel it's so cool.
Can create colors from white light.
You have to have a really, really, really thin film.
I'm going to make one.
So this is my film,
but it's not like this.
It's like this.
'Cause it's really thin,
like on the order of a few dozen molecules,
this is not a few dozen molecules.
This is probably trillions of molecules thick.
But imagine this is a few dozen molecules thick, so thin.
So it could be like a coating of some kind of paint.
It could be a bubble film.
It could be oil.
If you go search an oil spill on a road,
it's almost the same colors right?
We got that yellow, the magenta blue, but not red,
similar to bubbles suspicious huh?
Yeah so what you're seeing there is white light
and you know what white light is, right?
It's a combo of all of the colors,
I've got white light coming in with all all of its colors
onto my film.
And then some of it reflects off of the top of the surface.
Like so,
and some of it goes to the surface like this.
(chuckles)
This is great.
So some of this goes into the film
and reflects off of the bottom surface of the film
and then comes back out over here.
And now we have to remind ourselves of the fact that
light has a wavelength light is a wave.
You can't actually cut light waves in real life,
but this is going to for our purposes.
It looks like this
wave wave, wave, wave wave.
It's a wave of electromagnetism.
It's not like water or anything.
It's an electromagnetic wave.
So light has these peaks and troughs.
And if you want to measure how big a light wave is,
Yeah you can measure how tall it is.
That's the amplitude.
But another important way, especially for light
is to measure from the, up to the, down to the ups.
So this distance right here, here is the wavelength
or this trough to trough form, you know, whatever
that's one wavelength.
And the reason it's so important to know the wavelength
is because the wavelength is different,
for different colors of light.
So like blue has a shorter wavelength than red for example.
And I'll tell you
when you can recall that little fact later
because it's important.
And the other thing that's important is that
wavelengths of visible light are so small.
You've heard of nanometers,
visible light is a couple hundred nanometers.
If there were a bubble that small,
it would be way too small for our eyes to see
super tiny.
So when the film is similar and thickness
to one of these wavelengths,
then you can get interference.
So this is it. This is almost the end of the explanation.
So you have light coming in and reflecting off the surface,
got that.
And then this one that went into this film went farther.
So they were traveling together beforehand.
They're in sync like this
and the crests and troughs are matched up.
And then now this one's traveled farther.
So now they're out of sync. Get it.
So now you can see that this trough is matched up
with this crest, and those will cancel
because they're out of sync.
So some of you are astute
and you're wondering why all the waves started together
beforehand.
Why were they all in sync
and the answers that they weren't,
unless it was light from a laser coming in,
then the crests and troughs of a light beam
coming in are not coherent.
They're not lined up
and they're not all the same wavelength,
but when you have white light, like light from the sun,
you do have a bunch of random waves
and they're all randomly lined up.
But you do have so much light that you have pairs of waves
that are lined up
and you can get those pairs canceling.
So that's exactly what happens because
they meet up and you get canceling of colors.
Now is the time. Now you can recall
that the different colors of light
have different wavelengths.
So depending on what angle this light beam is coming in,
or depending on the thickness of the film,
then you'll get a certain distance
traveled by the light that went into the film.
And that tells you how far off these waves
are from each other.
And then that will determine which colors get blocked
because all the colors have different wavelengths.
So if you're off by like, say this much,
then you wouldn't get whatever color this is.
Maybe say, this is yellow. You wouldn't get it canceled.
It would get a little bit enhanced actually,
but it wouldn't get canceled,
but maybe you have blue light,
which has a shorter wavelength like this.
And if you're off by this much,
then you will end up getting canceling of this blue light.
This would be much more clear with an animation.
The point is if the waves are so far off,
that they're half a wavelength away like this.
So rather than having the crests and troughs matching up,
they're overlapping where this crest is meeting this trough.
Then they end up completely canceling that color.
So for certain thicknesses or certain angles,
you get red canceled out
or you get blue canceled out or yellow.
So we're almost about to see why you never see red
in a bubble.
So if you cancel, say blue,
if you take blue out of white light,
then you end up with something that looks kind of yellow.
If you cancel red,
you take red out of white light.
You'll end up with kind of a green, blue turquoise,
or as they say cyan.
And if you take yellow out,
then you end up with a magenta color.
So the only colors you can see in a bubble
are the result of subtracting out one color.
So you never get red because
you would have to cancel out yellow, orange, green,
blue, indigo, violet,
all the rest of the visible spectrum.
In order to see red,
you can't get pure blue either.
You can't get pure anything
because you're seeing just this result of color mixing.
And if you've ever done subtractive color mixing,
which digital artists do or people who edit photos or videos
with white balance,
they know subtractive color mixing,
it's the CMYK versus RGB,
which is where you add the colors.
So it's kind of like the opposite of a rainbow.
A rainbow is different because you're
taking white light and you're just spreading it out
so you can see all the colors.
So could you ever get red bubble?
Well, maybe we could use all of what we just learned
and use like a different material with like an array
of different indices of refraction that
determines how far each of the waves go.
But I don't know.
If that seems really complicated,
but there is an easier way.
If you take a bubble into the dark and shine it,
just red light on it,
you'd be able to get a red bubble.
But what you would see would be stripes
because red light is getting canceled and enhanced
and then canceled enhanced
because you're seeing the thickness of the bubble
where the red is getting canceled.
So this tells you something about the thickness of the
bubble.
It gets thicker as you go down because of gravity,
the liquid is drooping,
and then you end up with red
that's one half a wavelength off,
and then two wavelengths and then three half wavelengths
and so forth.
You can even see the thickness of a bubble in white light.
You just end up getting stripes of color.
It's really pretty.
I hope you enjoyed this video because I enjoyed
playing with bubbles.
Where is my bubbles?
Okay so I have a bit of a big momentous announcement
to make
if you want to stick around for another minute or so,
but first, as many of you know,
I've been part of PBS Digital Studios
for the last five years.
That's a hint about my announcement
and they have a bunch of other amazing shows.
And one of their newest programs aims to help us
make sense of the strange year. That is 2020,
it's called Self-evident.
And it's a new show hosted by Danielle Bainbridge,
from Origin of Everything
and Ali Mattu from the Psych Show.
It's a docu-series that tackles music,
activism long haul trucking,
finding love in the lockdown.
It's great. The link is in the description
and the big announcement is
that I'm actually ending my partnership with PBS,
but with love,
I've decided to continue this channel independently.
And I'm so excited and super nervous.
It's been such a great experience
being part of the PBS community
and not to mention my parents were thrilled.
They did not get this whole YouTube thing before that.
So you can still expect my physics videos
and the physics one series is going to continue.
But mainly now with the support of just my Patreons
over on Patreon.
So shout out to all of you on Patreon.
Thank you so much for your support so far
and thanks a million to PBS
and thank you to you all for always watching
and for happily physic-sing with me. So happy physic-sing.
(upbeat music)