How do optical illusions work?

You've seen optical illusions before. It's the picture that might be a goblet or might be two people kissing. Maybe you've seen those wavy lines that seem to pulse when you look at them, or lines that look like they're different lengths but a ruler shows they're the same. So, what's going on here? How do optical illusions work, and why do they trick us like that?  

Understanding optical illusions

There are three basic types of optical illusions: literal illusions, physiological illusions, and cognitive illusions. Here's what we're talking about:

• Literal illusions: These are pictures that look like other pictures. The young girl/old lady picture is like this. So are photomosaics, which are big pictures made up of lots of little ones. 
• Physiological illusions: These are the twisty, pulsating ones that make you feel weird. These work by overwhelming your senses with a lot of high-contrast colors and lines, lines, lines, until your poor eyes can't take anymore and you start seeing movement or odd effects.
• Cognitive illusions: These are really cool because they take advantage of the wonky way our brains work. More on this in a bit, but seeing things isn't done just with your eyes, but with your brain. These illusions trick your brain into seeing stuff that isn't really there, like the gray spots between the black squares or the goblet/kissing people one.

Mechanisms behind optical illusions

Okay, quick rundown on how we see stuff:

1. Light goes into each of your eyes, which trips light-sensitive cells and sends nerve impulses back to the brain.
2. In the brain, the slightly different pictures from each eye get superimposed and made into a 3D image. 
3. Only a small part of your field of vision actually sees stuff. This is the fovea, which is smack in the middle of each eye's vision field, about the size of a quarter held at arm's length. Images from the fovea are clear, everything around them is blurry, and there are. . .
4. Blind spots. The bit of your eyeball where the blood vessels poke through is a place where you have no vision cells. You don't notice it, but there's a blind spot at about 10 and 2 o'clock high from your field of view.
   
   

All you need to remember from this is that it's complicated. Because it's so complicated, your brain has to do a ton of work to make a sensible image out of the chaos it's getting from your eyes. The way it does this is simple: It cheats.

The brain uses a ton of tricks to make the images you see. Your eyes sweep all over the scene in front of you, for example, which drags the fovea all over and gives you brief snapshots of clear spots in your vision. Your brain holds those clear pictures in its memory and stitches them together to create the illusion that your whole field of vision is in focus. Likewise with colors, which you can only really see in the center, plus low-light stuff, which comes from your peripheral vision, and so on.

Combining all of this is as much make-believe and simulation as actual sensory input. The brain kind of knows what to expect in the world around it, so wherever it's in doubt about what it's seeing, it just kind of guesses and inputs the expected images without letting you know it's done it. Cognitive optical illusions work by faking out this system and tricking your brain into seeing things that aren't there.

Real-world applications and some cool examples of optical illusions

Optical illusions are cool as novelties, but they also have practical uses. In a way, every time you look at a sketch or even a photograph, you're looking at an optical illusion. It is, after all, a 2D flat image that looks like it has depth. Movies are also an illusion, since they're basically a flip book of slightly different pictures that zip past 24 times a second.

Another place you might notice optical illusions is in the movies themselves. Peter Jackson's Lord of the Rings, for example, was shot with a technique called forced perspective. This involves making things oddly out of scale to make the hobbits look small while the other characters look bigger. They did this in Casablanca, too. That airplane in the background was actually a flat cardboard prop about half the size of a real plane, with the crew played by actors just 4 feet tall to make it look big and far away.

In the real world, engineers have found uses for optical illusions. If you've ever driven up a gentle grade, you know your car will lose speed if you don't press the accelerator a bit harder. But if the upward slope isn't obvious, people won't accelerate; they'll lose speed, slow down, and cause traffic jams. To solve this problem, artists at Musashino Art University in Tokyo are toying with the idea of painting stripes on walls next to the road to artificially exaggerate the appearance of the grade, which in theory will encourage drivers to step on the gas.

Your eyeglasses and contacts are also playing a bit of a trick on your eyes. By bending light in a precise way, they can effectively create an image that would be out of focus for anybody but you. But because it is you, the errors introduced to the focus just happen to overlap with the opposite blur factor in your vision, creating an in-focus image.

Optical illusions are helpful and cool

Optical illusions are fun, but they also teach us a lot about how the eyes work. By using this knowledge, eye care professionals can craft a pair of lenses for you that open up a whole new world of clear vision. Get started on better vision today by finding the right contacts you needhttps://www.1800contacts.com/eyesociety/what-is-the-difference-between-expressexam-and-an-in-person-eye-exam and putting optical illusions to work for you.

FAQ

Can optical illusions cause eye strain?

It's not common for optical illusions to cause any problems with your eyesight, but to be safe, you should stop looking if your eyes feel tired or you get a headache.

Are some people more susceptible than others to optical illusions?

Nobody really knows why some people are more prone to seeing optical illusions than others. People with red-green colorblindness, for example, were famously good at seeing through camouflage during WWII. For other types of optical illusions, there's still a lot of work to be done.

How do optical illusions affect depth perception?

Many optical illusions work by tricking our sense of depth. This can be for fun, but in a sense your glasses do something similar by pulling focus closer or farther away.

Can vision correction affect how you see optical illusions?

Most optical illusions can't work on you if you have blurred vision. If anything, good vision correction will make it more fun to flip through a book of illusions than it ever was before.