It’s pretty hard to fight an opponent you can’t see. That’s what makes Invisible Woman an invaluable member of the Fantastic Four. She can disappear in the blink of an eye and take out entire factions of enemies, all while her adversaries kick and punch empty air.
Characters with invisibility powers — Marvel Comics’ Invisible Woman (aka Sue Storm), H.G. Wells’ Invisible Man and The Incredibles’ Violet Parr — can do all sorts of cool things. Escape embarrassing situations. Snoop on other people. Go places where they’re not allowed.
Short of a theoretical invisibility cloak, that kind of lurking is impossible for us. But some animals can hide in plain sight by turning nearly transparent.
Invisibility is useful for animals that spend a lot of time in places with no good hiding spots, such as the air or open ocean, says Kate Feller, a visual ecologist at Union College in Schenectady, N.Y.
The secret to invisibility is all about light. “The way we see things is when light is reflected back off of an object onto our eyes,” Feller says. The more light that can pass through something undisturbed, the harder it is to see that thing.
Light can’t easily pass through most animals, but those with nearly invisible body parts have tricks to get around this hurdle.
Animals get their colors from pigments, compounds that appear as different colors depending on which wavelengths of light they absorb and which they reflect. Green pigments, for instance, reflect green light and absorb all other colors. Invisibility requires getting rid of these colorful compounds.
Most baby fish lack pigments at first, helping them hide from predators. Some species stay see-through their whole lives. One is the ghost catfish (Kryptopterus vitreolus). With a body that’s fully transparent from gills to tail, it looks like a floating head from afar.
It’s hard to hide every body part, though. Eyes need light-absorbing pigments to see. And even clear stomachs will show up if animals eat colorful food. A ghost catfish eats only small bits of food and its long, thin, tubelike stomach stretches out a meal, helping the organ stay hidden, says Qibin Zhao, a physicist and materials scientist at Shanghai Jiao Tong University in China. Blood can’t be transparent either due to the iron-rich hemoglobin molecules that carry oxygen. So up close, the ghost catfish’s blood vessels are still visible.
But even pigment-free objects are not entirely invisible. Light travels at different speeds through different materials so will bend or bounce when it goes from one material to another. That can create a shine, like sun reflecting off a puddle.
That’s not a big problem for a glasswing butterfly (Greta oto): Its wings have fewer reflective scales than other butterflies’ wings. The wings are also covered in tiny wax bumps that make light’s transition from air to tissue more gradual, so light passes through the wings mostly undisturbed.
Invisible Woman uses a completely different vanishing trick than real-life transparent animals. Instead of letting light pass through her, she bends lights rays to completely avoid her body. The rays never even touch her, so they are not reflected or absorbed, making her invisible to any onlookers.
In 2006, scientists proposed that this same logic could be used to create an invisibility cloak. But in practice, a material that perfectly bends light rays is expensive, hard to make and can divert only a narrow band of light at a time. That’s just enough to make a single-colored object disappear but not an entire human.
And there’s another problem: With no light getting into the cloak, a person inside can’t see out. Luckily, Sue Storm has the ability to use cosmic rays to see while she’s invisible, allowing her to protect the world from supervillains like Doctor Doom.
