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As a SciShow viewer, you can keep building your STEM skills with a 30-day free trial and 20% off an annual premium subscription 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.

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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]