Although illusions are by definition not real, scientists are increasingly finding ways to use them to make an impact on the real world. Here are 10 practical applications that use or control illusions, from warships to virtual reality to Michelangelo's David and the Statue of Liberty.

1.

MP3s

All MP3s are based on illusion. An MP3 is a product of “lossy audio compression,” in which parts of the audio signal are discarded to shrink a sound file yet cause zero to minimal perceived loss in the sound quality for the average listener.

How does MP3 compression work? It eliminates generally inaudible sounds. For example, if two sounds are presented simultaneously very close together in time, then depending on their frequency and intensity, the brain may only perceive one of them, a phenomenon known as masking. "Lossy audio compression absolutely involves an illusion," says perceptual and cognitive psychologist Diana Deutsch at the University of California, San Diego. "You can reduce the amount of information in a signal and the brain fills in the gaps."

After removing sounds that people cannot generally hear and then using traditional compression methods that encode redundant information to save space, MP3s can reduce the size of a sound file by 90 percent or so.

woman listening to music
Image: Thinkstock

2.

Futuristic lightbulbs
In 2012 cognitive neuroscientist Stephen Macknik, now at the State University of New York's Downstate Medical Center in Brooklyn, and his colleagues discovered an illusion that allows a light to essentially flicker off for 13 percent of the time yet appear as bright to the human brain as if it was on the entire time. By creating a lightbulb that exploits this illusion, the researchers hope to create bulbs that save up to 20 percent on electricity bills.

Researchers have analyzed the flickering of lightbulbs for as long as they have existed. For instance, Thomas Edison found that bulbs flickering 60 times a second appear to be constant light sources, says Macknik (who also serves on Scientific American MIND’s board of advisers).

Macknik and his colleagues discovered that perceptions of brightness were not only linked with the duration of light flashes, but also with their periodicity. Their findings suggest that if 67-millisecond pulses of light were alternated with 10-millisecond periods of darkness, people would nonetheless perceive these light pulses as one and not as flickering.

Macknik suggests this discovery could lead to lights that save energy and last longer. The savings might be dramatic—roughly a quarter of all electric power is used for lighting, according to the U.S. Energy Information Administration. The invention is patent pending, and Macknik has received a National Science Foundation grant to research the physiological underpinnings of this illusion in the brain.

woman listening to music
Image: Cornell

3.

Trimming waistlines
Scientists have long known the Delboeuf illusion can make people misjudge the size of identical circles surrounded by larger circles. The more empty space around the inner circle, the smaller it appears.

Similarly, in 2006 researchers Brian Wansink and Koert van Ittersum found that larger plates can make the same amount of food appear smaller and vice versa. The scientists also noted that the average size of a dinner dish in the U.S. has increased 22 percent since 1960, perhaps helping to explain why people now eat more per sitting.

This finding suggests that using smaller plates might be one simple step to trim waistlines and waste less food. The Small Plate Movement now aims to get families and restaurants to use smaller dinnerware. Indeed, researchers Steffen Kallbekken and Håkon Sælen found that Scandinavia's largest hotel chain reduced food waste by 20 percent by reducing plate size as well as by posting signs encouraging customers to help themselves to buffets of food more than once per meal in order to nudge them to not overload their plates on the first visit to the steam table.

woman listening to music
Image: Honou via flickr

4.

Traffic jams
Dips in the road, where the pavement shifts from downhill to uphill, often lead to traffic jams. The visual illusion of a vertical gradient tends to lead drivers to misjudge a road’s slope. This means that at sags drivers often fail to realize they are going uphill and hence drive without accelerating to maintain speed, causing traffic slowdowns and jams.

Scientists in Japan suggest that patterns of stripes on walls near roads might help counteract this illusion, helping smooth traffic flow, says researcher Akiyasu Tomoeda at Musashino University in Tokyo. The researchers are testing whether such patterns might work using driving simulators, he says.

woman listening to music
Image: Carlos Cruz via Wikimedia Commons

5.

Forced perspective
Forced perspective relies on an optical illusion to make objects appear closer, farther, larger or smaller than they actually are. For instance, in film, in the final scene of Casablanca the escape airplane was actually a miniature, flat cutout whereas the extras working on the craft were little people, which helped create the illusion of size. Modern film versions of forced perspective include giant Hagrid in the Harry Potter movies and the dwarves and hobbits in the Lord of the Rings trilogy. "Forced perspective works because the 3-D vision system in humans is crap," Macknik says. "We use a flat retina, a 2-D 'device,' to see in 3-D and therefore must rely on perspective clues to determine size and distance."

In sculpture, classic examples of forced perspective include Michelangelo's David and the Statue of Liberty where the illusion helps make their proportions seem more natural when viewed from below. Forced perspective is also used in amusement parks such as for Cinderella's Castle at Walt Disney World and the new Hogwarts Castle at Universal Studios to make those structures appear more impressive.

woman listening to music
Image: OhMaGif.com

6.

Video
Animation, films and online video all rely on an optical illusion known as the phi phenomenon, where still images viewed in rapid succession create the illusion of continuous motion. For instance, movies flash at least 24 still images per second at viewers, flickering that is fast enough to create the illusion of smoothly moving images. "The phi phenomenon works because the visual system doesn't really see motion at all—it sees edges at different positions in space and makes the assumption that they are moving," Macknik says. "So when you blink a box in one position and then another, the visual sees it as motion because that's how it sees real motion, too."

7.

Virtual reality displays such as the Oculus Rift are based in part on decades of research into flight simulators, which in turn rests on centuries of knowledge from the art world on how to use optical illusions to fool the eye and mimic depth. "Our brains have always used perspective clues to see depth, even before art existed," Macknik says. "Artists simply figured out what those rules were and utilize them to make things look to be at a different depth or size than they really are."

For example, using linear perspective to create the illusion of depth, lines converge and objects are drawn smaller in size to appear as if they are receding from the viewer. Using aerial perspective to make items seem as if they are farther away objects may be drawn blurrier and with a tinge of blue to mimic the effect atmospheric haze has on the appearance of distant objects. Using shading painters can imitate the effects of light and shadow on solid objects to create the illusion of form.

Flight simulators add effects such as tilting, shaking and sound to make the illusions they display seem more realistic. Virtual reality also seeks to re-create experiences such as touch and space using wired gloves and omnidirectional treadmills as well as to enhance illusions by simulating interaction between people and their virtual surroundings.

woman listening to music
Image: PLOSone

8.

Dazzle camouflage
Dazzle ships were warships painted in garish stripes and other bold patterns during both world wars to confuse the enemy. The idea was that dazzle camouflage would disrupt enemy perception of the range, heading, size, shape and speed of the ships and hence reduce losses from torpedo attacks by submarines. A commander of a German U-boat could only pop his periscope up for about five seconds maximum, says vision scientist Nick Scott-Samuel at the University of Bristol in England—any longer, and the periscope would leave a wake visible enough to leave the U-boat vulnerable to getting hit by depth charges." In those five seconds or so "he'd have to make the calculations needed for a torpedo to hit a moving target. That's very hard to do, and the goal of dazzle patterns was to interfere with perception further."

In experiments involving moving items on video screens Scott-Samuel and his colleagues found that dazzle patterns could indeed distort the perception of speed, most notably at high speeds. The researchers suggest that in a rocket-propelled grenade attack against a Land Rover moving at 90 kilometers per hour at a distance of about 70 meters, a typical distance for such an assault, dazzle camouflage may help the grenade miss where it was aimed by roughly one meter, which could be the difference between life and death for the vehicle occupants.

woman listening to music
Image: Thinkstock

9.

Telephones
Smartphones and other telephones all rely on an auditory illusion known as the missing fundamental. Complex sounds such as speech or music are composed of many different frequencies of sound—for instance, 100, 200 and 300 hertz. In that example the second and third frequency are multiples, known as harmonics, of the first and lowest frequency, called the fundamental. "With the missing fundamental illusion one can still hear the fundamental even if it is removed, as long as you have a sufficient number of adjacent harmonics in the sound," U.C. San Diego’s Deutsch says.

Telephones usually only transmit sounds between about 300 and 3,000 hertz due to their small, low-quality speakers. The fundamental of adult voices is generally below 300 hertz and that of the typical male voice is about 100 to 150 hertz, but over the phone we usually do not confuse adults with children nor men with women. "The brain reconstitutes the missing fundamental so you can hear pitches that are not actually there," Deutsch says. "Why does the brain do that?" she asks. "Well, when sounds travel they bounce around a lot and the different components of a sound become lost or reduced depending on walls and various objects that the sound bounces into. So as long as we make use of the missing fundamental illusion, we can recognize a sound even though it might have a different frequency composition from one circumstance to the next."

Modern telephony takes advantage of this illusion by transmitting a smaller bandwidth of frequencies than is actually in any given conversation. The fact that phone systems transmit less information makes it possible for them to carry more calls on a single line, making them more cost-effective.

woman listening to music
Image: PerceptionWeb.com

10.

Swimsuits
Why do high-cut bathing suits make legs look long? It's the high-cut leg illusion, says Kazunori Morikawa, now at Osaka University in Japan.

In a study, Morikawa asked 76 undergraduates to look at drawings of a woman who wore either a high-cut bathing suit or calf-length tights. The students significantly overestimated the length of the woman's legs when she was in the swimsuit.

This illusion is a variant of the famous Müller–Lyer illusion. The most common version involves two lines of identical length, each with diagonal lines jutting out from their ends like fins or wings. The line with the outward-jutting wings looks longer than the line with the inward-jutting wings. Morikawa says the Y-shaped contours formed by the leg openings of a high-cut swimsuit and the inner contours of the legs may evoke the Müller–Lyer illusion.