- This is a type of shower gel called Foamburst.
It comes out blue, but if you agitate it, it turns
into a foam and it changes color from blue to white.
This is some water that's been stained dark green,
there's some soap in there.
If I shake it,
it turns white.
This soggy picture of a rhino turns white as it dries.
This Pernod turns white, when you pour it into water
as does this bath emollients.
If you buy a fog machine
you'll be disappointed to discover the only color
of fog you can buy is white.
Clouds are white.
Lots of things are white.
Why is that?
And what's it got to do with wet t-shirts and Buddha boards.
For a while now, I've been trying to figure out
how Buddha boards work.
A Buddha board is a white piece of card
and you can paint on it with water.
So a brushstroke of water turns the card black
so you can create artworks
but the artwork you create is temporary
because when the board dries
it returns back to its white color.
The superficial explanation is quite simple.
You have a black piece of card
and then you have a special white coating on top.
It's special because it becomes transparent when wet.
So it reveals the black color underneath
which explains where you can have more complex images
like this color picture of a rhino.
You can take any picture you like
and coat it with this special layer.
But the question is, how does the layer work?
Why does it become transparent when wet?
And I think I have the answer to that.
Let's consider the shower gel to begin with why is it blue?
Well, if you take white light
and remember white light is just all the colors
of the spectrum mixed together, and you shine it
onto the shower gel, then the shower gel absorbs some
of the red and green parts of the spectrum.
And the blue parts are reflected back
into your eyes or more correctly
it's scattered in all different directions.
Some of which ends up in your eyes.
Scattering is just reflecting by the way,
except that it's off a rough surface
instead of a smooth surface.
So it doesn't obey the laws of reflection.
Either that or the light is bouncing off molecules
or particles randomly distributed within a substance.
Like we've got here with the shower gel, but anyway
that's why the shower gel is blue.
So why does it turn white when it foams up?
Well, let's take a cross section
through the foam because of all those bubbles
you've got all these different surfaces
all oriented in different directions.
And when light lands
on one of these surfaces, two things happen.
Some of the light gets reflected.
Some of it gets transmitted.
The transmitted light is refracted
but that's not so important here.
The important thing is the light bounces
around inside the foam.
Some of it getting transmitted,
some of it getting reflected.
And eventually of this chaotic bouncing around
inside the foam, some of it will bounce back out
towards your eyes and crucially, this scattering of light
as it's called happens to all the colors of the spectrum.
So the light that reaches your eyes is that full spectrum
of colors mixed together, it's white.
And that's why foam looks white.
You might think at this point, well, hold on
the shower gel still has that blue dye in it.
So when some of the light is transmitted
through those thin layers of the shower gel
the red and green parts of the spectrum should get absorbed
by that dye and the blue parts scattered.
So why doesn't the foam look a little bit blue?
And the truth is that, well, yeah, it does.
It looks a tiny bit blue.
It's just completely overwhelmed
by the scattering of all the colors of light
through reflection and transmission.
And that makes sense because foam is mostly air
like this big thick chunk of shower gel here
looks pale blue.
So if you spread that out and make it mostly air
then that handful of very thin surfaces
that the light passes through in the foam state
isn't gonna experience much of that scattering
and absorption from the dye itself.
Here's another way to think about it.
I've got some blue liquid here and if I move the camera
suddenly it turns completely bright white.
Actually, what you're seeing is the reflection
of my light source this light panel here,
in other words, you're seeing glare.
So by moving the camera
I can go between these two extremes.
One way you can see the true color of the liquid
and one way you can't see it at all.
What foam does is kind of average out these two extremes
because the surface of the foam
is oriented in all these different directions.
There's no one sweet spot where you get glare.
You get a little bit of glare from the entire foam surface.
And it turns out that when you average out the color
of the liquid with the glare of reflection from the light,
actually the glare of reflection from the light
mostly winds out and the foam is white in appearance.
So in simple terms, the reason lots of things are white
is because of scattering.
It's white clouds are white
but the details are often quite different.
For example, the sky is blue
for a different reason to shower gel is blue
or at least this shower gel.
With clouds, you've got these droplets of water
and they're acting like the surface
of the foam or the surfaces within the foam.
In other words, the light is bouncing around, off and in and
through those droplets of water in all different directions.
Eventually that light finds its way to your eyes.
And you see it as a white cloud.
The example of Pernod turning cloudy
when you add it to water is even more interesting.
There are these essential oils that are dissolved
in the alcohol of the Pernod that aren't soluble in water.
So when you add the Pernod to the water
the concentration of alcohol goes down.
The essential oils are no longer soluble in that solution.
So they drop out of solution and form these tiny droplets.
Just like if you have a layer
of oil on top of water and you shake it, you end up with
droplets of oil suspended in the water.
Though in that case
the droplets start to join together into bigger droplets,
they rise up to the top and they all smush together
into one big mass of oil.
For some reason that doesn't happen
in the case of the aromatic molecules in Pernod
they remain suspended as tiny droplets.
They don't join together and they don't fall or rise.
And from what I can tell nobody knows why.
What's all this got to do
with the workings of a Buddha board.
Well, let's have a look at frosted glass
as an intermediary step.
Frosty glass is frosted
because the surface is rough and not smooth.
So when light hits the frosty glass
it bounces off those different facets
of the glass that are oriented in different directions,
otherwise it scatters.
That's why it looks slightly less transparent
and slightly more white.
What if you could fill in those gaps
and smooth out the surface?
Well, you can do that with sticky tape.
The glue of this sticky tape will fill in all those gaps.
And look, the glass is no longer frosted.
If you work in an office
with cubicles and they're separated by frosted glass
and you wanna see through the frosted glass
just put some sticky tape up there
and you'll be able to see through.
This only works because the refractive index
of the glue in the sellotape is very similar
to the refractive index of the glass,
so that when light passes from one substance to another
it's mostly transmitted and it's hardly refracted at all.
The same thing happens with fabric.
So fabric is made of lots of really thin fibers
all pointing in different directions.
And so they scatter light in the same way that foam does.
But if you wet fabric
then the water gets in between all the fibers,
it fills in all those gaps.
And because they're refractive indices of water
and fabric are much closer together
than the refractive indices of air and the fabric
it reduces the amount of scattering
much more of the light simply passes
through or is absorbed by the fabric,
which is why the fabric appears darker when it's wet.
It also makes the fabric more transparent,
which is why if I pour water on this t-shirt
you can start to see my nipples.
So here's my hypothesis about the Buddha board,
that coating on top of the black card
must be scattering light.
And that's why it's white.
So three important properties for whatever that coating is
it must be really rough so that it scatters light.
And we can kind of see that under the microscope here.
It needs to be transparent, which is to say
if the surface wasn't rough
you'd be able to see through it really well.
And the third thing is it probably has a refractive index
that's really close to water so that when you do wet it
the scattering is reduced almost to zero,
and you can see right through to that dark layer underneath.
I wish I could tell you what that chemical substance is
on the surface of the Buddha board
but I can't find it anywhere online.
I suspect it's a trade secret, but if you know
or you have any ideas, let me know in the comments.
One final thought that might be on your mind is,
well not all things that appear white
have a roughness to them like a white car for example
or a wall that's been painted white.
Well, the vast majority of white paint is made white
through the addition of titanium dioxide
and titanium dioxide is a crystal that isn't white.
It only becomes white when you crush it into a fine powder,
of course, because of scattering lots of tiny crystals,
all oriented in different directions, you get scattering.
In fact, the refractive index of titanium dioxide is so high
that you can add this fine powder
as a suspension to a liquid
and it will still appear white because the refractive index
between these two different things is still quite large
which is why paint still appears white even when it's wet.
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