Thanks to Brilliant for supporting this SciShow
video! As a SciShow viewer, you can keep building
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at Brilliant.org/SciShow. Believe it or not, there are people who can
see what's invisible to most of us. I'm not talking about some sci-fi superpower
like X-ray vision, but how it actually works isn’t all that
far off. The average person sees millions of colors
from violet to red, but those colors don’t make up all of the
light there is. Because of the structure of our eyes, there
are some wavelengths of light outside of the range that triggers our vision,
making them impossible to see. Not for everyone, though! Thanks to certain genes or conditions, some people can see what’s invisible to
the rest of us. [♪ INTRO] But before we can start seeing the invisible, let’s talk about how seeing the regular,
visible stuff works. When light bounces off an object and enters
our eye, it passes through the cornea first. That’s the outer, dome-shaped structure that bends light toward the center of the
eye.. Some of this light goes through the pupil, which gets bigger or smaller in different
settings to let in more or less light. Then this light passes through the lens, a part of the inner eye that helps focus it
further. Finally, the light hits the retina at the
back of the eye: a layer of tissue covered with special cells
called photoreceptors. Now, our photoreceptors can only respond to
certain wavelengths of light, which for humans, is between 380 nanometers
and about 700 nanometers. Our photoreceptors intercept those wavelengths
of light and convert the energy in that light into
electrical signals. Then these electrical signals travel through
the optic nerve to the brain… and the brain turns them into an image of
the world! So there are lots of steps that collectively
make vision possible. Most of the time, if there’s a problem or
variation within any of these structures, that’ll make it harder to see. But now and then, some variations actually
reveal the invisible. Light waves can have many different wavelengths, and those wavelengths make up the spectrum
of visible light that we can see. Outside of the visible spectrum of light, there’s a whole realm of ultraviolet, X-rays,
and gamma ray radiation. And as cool as it would be to have X-ray vision
or see UV light like a bee, it’s actually a good thing we don’t, because UV light can be just as damaging to
our eyes as it is to our skin. Which is why there are barriers to that light
getting too deep into our eyeballs, mostly within the lens, which has yellowish pigments that absorb UV
rays before they go any further. Kind of like built-in sunblock, but only in
our eyeballs. But some people are missing a lens in one
or both of their eyes, so that UV doesn’t get blocked. Lacking a lens is called aphakia. And in aphakic people’s eyes, UV light can
sail straight through the eye and trigger the photoreceptors on the retina. Individuals with this condition have said
that UV light looks whitish-blue or -violet to them. One of the most famous people with aphakia
was the artist Claude Monet, who had the lens of one eye removed as a treatment
for cataracts. Afterward, he complained about seeing everything
with a bluish tint, as well as other problems with his vision. His paintings from the time after the surgery
give us a window into what this might have looked like for
him, too. For example, in this painting, the petals
of white lilies have a bluish tinge, which is likely the UV light he saw reflecting
off of them. And while aphakia might seem like a superpower,
it has a downside. The lens focuses light onto the retina, so
not having a lens results in blurry vision. Which is why we should leave seeing UV light
to the bees. So the colors of light that appear bluish-violet
to us have very short wavelengths, and UV’s wavelength is even shorter than
those, so we can’t see it. And on the other end of the spectrum, the longest wavelengths we consider visible
come from red light. And wavelengths that are longer than about
800 nanometers are what we call infrared, and are generally
undetectable to the naked eye. Longer wavelengths mean less energy, so infrared waves don’t usually have enough
energy to trigger the chemical reaction in our eyes that turns
light into electric signals. But throughout the 20th century, several scientists
who’d investigated the range of their vision in lab experiments reported
that they could see some infrared. The question was… how? In 2014, a group of scientists who’d been
seeing green flashes while working with an infrared laser decided to get to the
bottom of what was going on. To do that, they shined pulses of infrared
laser light into volunteers’ eyes, and all of them were able to detect a visible
light signal. But the weird thing about it was that the
color they saw corresponded with a wave frequency that was about double
the frequency of the laser. So when the laser beam had a frequency of
1000 nanometers, they saw it as light with a frequency near
500 nanometers, which looks green. That suggested that when the laser was pulsing
quickly, the photoreceptors in the eye would process two pulses of infrared
light at once, doubling the amount of energy that hit that receptor and essentially
tricking it into going off. So the secret to becoming visible lies in
teamwork, at least for infrared waves. We’ve established that there are ways to
see light beyond either end of the visible spectrum. But there are also people who can see extra
colors within that spectrum. See, there are two kinds of photoreceptors
in our eyes: rods, and cones. Most humans have three different types of
cones, and each one contains different pigment molecules
that absorb light. Depending on which pigments it contains, each
cone is most sensitive to a different wavelength of light: either
blue, green, or red. So as different colors of light hit our eye, they trigger different combinations of these
cones. And these combinations create every color
we can see. For most sighted people, that’s millions
of colors! But now and then, someone ends up with a fourth
cone. This can happen because the genes for red
and green cones are found on the X chromosome. So people with two X chromosomes have two
copies. And if there’s a mutation on one of the
X chromosomes, it can create a new type of cone containing a pigment molecule that’s
sensitive to a different color of light. This fourth color can vary from one case to
another, and it doesn’t always have an effect on
the person’s vision. The new pigment molecule they have could just
be a repeat of one of the others, or just not do anything at all. But in rare cases, these four types of cones
are triggered by four different colors of light and produce millions of colors that
can’t be made with just three cones. So, people with this condition, called tetrachromats,
can see a whole range of colors and shades that are completely indistinguishable
to most of us. It’s less that they see colors that are
invisible to us, and more that they can tell the difference between colors that look exactly
the same to most other people. Which means they’re probably way better at telling their black socks apart from their
navy ones. And while having four distinct receptor types
is above average for a human, tetrachromats have got nothing on mantis shrimp, which can have between 16 to 21 kinds of photoreceptors
in their eyes. Just, you know, to put things in context. All this is a reminder that what any one person
sees isn’t an objective representation of the
world. It’s just a window into the world, and quirks
of physics and biology can reshape that window and redefine what
we consider visible. It’s all about perspective! This SciShow video is supported by Brilliant. Yes, it’s supported by brilliant people
like you who continue to watch SciShow videos, but it’s also supported by the interactive
online learning platform with thousands of lessons to choose from in math,
science, and computer science. For example, there’s the Brilliant course: Geometry I that teaches you the ins and outs
of angles. With this course’s geometry puzzles, you
get to learn your own way and come up with creative geometric problem-solving
techniques. You might even think about this SciShow video in new ways after taking that Brilliant course. If you thought it was a feat to fit all of
our rods and cones in these little eyes, just imagine the incredible geometry involved
in making mantis shrimp eyes work. And after you take this Brilliant course,
it’ll be a lot easier to imagine. You can find it all at Brilliant.org/SciShow. That search will start you off with a free
30-day trial and 20% off an annual premium Brilliant subscription. Thanks for watching! [♪ OUTRO]
I was just going to share that video here. Beaten by 10 mn !