- 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
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