# Wonders of Reality 12 —“The Bridge of Death”

Here it is! The bridge of death! Oh, great! Look! It’s the troll from the “Lost Treasure”! What’s he doing there? No he’s the bridge keeper, the science
troll. He asks a traveller three questions. The one who answers correctly
may cross the bridge safely. What if I get a question wrong? Then he costs you
into the river of eternal shame. No, not that! And what does he ask about? In autumn, he asks about light and your favourite colours, unless the troll guidebook is lying to us. About light! Why didn’t you tell it right off? That’s
easy! Stop! Who would cross the Bridge of death must answer me these questions
three ’ere the other side he see! Ask me a question bridge keeper! I’m not
afraid! What is your name? it’s fairy Iridessa from the Pixie
Hollow. What is your quest? I’m looking for pixie dust. I want to help my fellow fairies. What is your favourite colour? Yellow and orange! No! What is your favourite
colour… according to the subtractive colour model! I don’t know that… Our world is colourful, but why are leaves yellow, apples—red,
flowers—blue and some fairies in the river green? He-he! Let’s start with light. Light is a tiny part of the electromagnetic spectrum that we see. If a lamp radiates
light of all visible frequencies it appears white. The paper also appears
white if it evenly reflects the light of different frequencies. Now let’s apply
some blue paint. What happens, eh? As you might guess, the blue pigment will
reflect the blue rays and some green ones, while the yellow and red will be
absorbed. And what colour do we get if we mix blue and yellow paints? It’s green! Right! And why? Simply because the yellow pigment reflects the yellow light and a
bit of the green and red as well, But it stops the blue light. The same thing
happens if we pass light through blue glass and then through a yellow one. Only “green” photons will survive. They bounce off the paper and we see them. The more
paint the more photons shall we subtract from the light. That is why we call this
colour model subtractive. It comes from the Latin subtraho.
So when we mix watercolours, we always get darker colours. We can only absorb
more photons without adding new ones by magic. So what’s the point in this model? It’s crucial! I have already mentioned pictorial art, but colour printing works
the same way! Let’s apply a layer of cyan colour, add magenta, yellow and finally some black paint to the paper. That’s the way colourful drawings and
photos are printed. You came out pretty well here! Iridessa hold on! We’ll get you
out of there! Stop! Stop!
Who would cross the Bridge of Death must answer me these questions three, ’ere the
other side he see! I will! You bet your life! What is your name?
Fairy Tinkerbell! What is your quest? I’m looking for dust for fairies from
the Pixie Hollow! What is your favourite colour… Hmm… According to the additive colour
model! Huh! Let me tell you about that! Colour is biology—not physics! The retina
of our imperfect eye is all draped with photoreceptors. The rods sense the
brightness, while cones… Well it’s not so simple about them. There are three types
of cones: first perceive the red colour, second sense green and third react to the
blue colour. But wait! Where are the cones for orange and teal colours? There are
none! Yellow light excites both red and green receptors. We call such a mixed
signal yellow. The ice sees no difference between mono- and polychromatic light.
What did you say? I mean we can’t distinguish real yellow light from a
mixture of red and green. Hence we only need three coloured
spotlights to trick the vision and show any colour. For one, orange or teal. And if
we mix red green and blue together, we get white. Only thanks to such
imperfection of the eye, projectors and monitors that show a pretty plausible
picture with millions of colours appeared. The screens glow lighting up red green
and blue dots. The light adds That’s why the model is called additive from Latin addo (add). So how can we write your favourite colour in the red-blue-green
model? Green is written like this: zero, 200, zero. Wait, what? Hey that’s unfair! you said it on purpose of fooling me! My answer was totally right! “Red blue green”? What a
dirty trick! It’s red green blue. Hey! Get me out of this mud and apologise! Right!
And get out Iridessa as well! She called the colour correctly! Yes she
didn’t tell you about the subtractive model but hey she’s not supposed to know that! After all, we’re not going to art class you know! The lightening struck our
Home Tree, that’s why we’re here! We came for pixie dust… Who would cross the Bridge of Death, must answer me these questions three… Go ahead! Might as well. Since you can’t scare anyone. [To Tink] More pressure next
time. You almost got him! Listen carefully I won’t repeat twice! What is your name?
The Empress of the wind dairy Vidia from the Pixie Hollow. What is your quest? Hunting for pixie dust and dying to fly again! What? Are you suggesting that I lie and
to take a mud bath with you? It’s true that I just want to fly! And yes, if I
prevail, I’ll share the loot with the others. Can’t wait to leave this swamp! What is your favourite colour… and what is its wavelength? What do you mean? Mono- or
a polychromatic light? Let’s say monochromatic! Tell me the wavelength in nanometers! It does not exist! A colour without wavelength? Hmm… I don’t think I
know that! Let me explain!
Every rainbow color has its wavelength, but wait a second! Where a pink? Magenta? And my favourite purple colours then? Not here! And not there either! does it mean that my favourite colour doesn’t have its wavelength?
Not quite. Tink said it right that colour is biology not physics. And your colour wheel is a big hoax. If we try to roll the rainbow in a ring, where are we supposed
to fit the radio waves? microwaves? X-rays? And other stuff? Oh well… since we don’t
see anything of this richness there is nothing left but to substitute all the
variety with a purple colour or rather with a mixture of red and blue. In other
words purple is “minus green”. Get rid of the green part and you get my favourite
polychromatic purple colour! And please do me a favour! Stop drawing rainbows with
purple colour there is none next time when you see a rainbow take a closer
look. You’ll find a violet stripe (unless you have problems with colour vision) but
not a purple one. Technically, raindrops may arrange in a
sophisticated way and after all reflections we might see a bit of pink
after all. But such an event is extremely rare. By the way, if you like science
fiction keep in mind that there are no purple lasers. Purple is always a mixture
of red and blue. Looks like I’ll cross the bridge after all. And you, my fair
troll, will sit in the… how did you call it? River of Eternal Shame! Why? why is it called so? Is this dirt unwashable? He-he! Ask Vidia! How am I supposed to know that? No no
don’t you… Hawk take you! It is not hard to wash it off. Unfortunately, our shame will be remembered till the end of time!