Category Archives: Illusions

Throwing You a Curve

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The break of the curveball illusion.

In baseball, a curveball creates a physical effect and a perceptual puzzle. The physical effect (the curve) arises because the ball’s rotation leads to a deflection in the ball’s path. The perceptual puzzle arises because the deflection is actually gradual but is often perceived as an abrupt change in direction (the break). Our illusions suggest that the perceived “break” may be caused by the transition from the central visual system to the peripheral visual system. Like a curveball, the spinning disks in the illusions appear to abruptly change direction when an observer switches from foveal to peripheral viewing.

Very Telling

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Magic and the Brain: Teller Reveals the Neuroscience of Illusion

“Tricks work only because magicians know, at an intuitive level, how we look at the world,” says Macknik, lead author of the paper. “Even when we know we’re going to be tricked, we still can’t see it, which suggests that magicians are fooling the mind at a very deep level.” By reverse-engineering these deceptions, Macknik hopes to illuminate the mental loopholes that make us see a woman get sawed in half or a rabbit appear out of thin air even when we know such stuff is impossible. “Magicians were taking advantage of these cognitive illusions long before any scientist identified them,” Martinez-Conde says.

There are some quotes from Teller, though these are obviously illusions, because Teller is the one who doesn’t talk.

Before long, they were performing Cups and Balls on Letterman. The trick became a centerpiece of their first off-Broadway show. “It was so liberating to be able to treat the audience like intelligent adults,” Teller says. Instead of engaging in the “usual hocus-pocus clichés,” the clear cups forced the crowd to confront the real source of the illusion: the hard-wired limitations of their own brains. Because people were literally incapable of perceiving the sleight of hand—Teller’s fingers just moved too fast—it didn’t matter that the glasses were transparent.

Color on the Brain

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Magenta Ain’t A Colour

When our eyes see colours, they are actually detecting the different wavelengths of the light hitting the retina. Colours are distinguished by their wavelengths, and the brain processes this information and produces a visual display that we experience as colour.

This means that colours only really exist within the brain – light is indeed travelling from objects to our eyes, and each object may well be transmitting/reflecting a different set of wavelengths of light; but what essentially defines a ‘colour’ as opposed to a ‘wavelength’ is created within the brain.

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Magenta is an “extraspectral” color. Sir Isaac Newton noticed that magenta did not exist in the spectrum of colors from white light when he played with prisms. But when he superimposed the red end of the spectrum on to the blue end, he saw the color magenta

Deeper Than it Looks

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My recent post on Haidinger’s brush reminded me of another optical phenomenon I have observed, and one that is a bit easier to see: the Pulfrich effect.

The Pulfrich effect is a phenomenon that gives the illusion of depth based on the response time of different light levels in the eye and how the brain interprets the delay. Lower light levels take longer to process, so if the image viewed by one eye is dimmer than the other, the signal from the dimmer view will lag in reaching your brain to be interpreted. With one eye darkened, something moving across your field of view will appear to get closer or further way.

When I first read about this, I consulted Wikipedia, which tells me

In the classic Pulfrich effect experiment a subject views a pendulum swinging in a plane perpendicular to the observer’s line of sight. When a neutral density filter (a darkened lens – typically gray) is placed in front of, say, the right eye the pendulum seems to take on an elliptical orbit, appearing closer as it swings toward the right and farther as it swings toward the left.

Well, gee, I work in an atomic physics lab. I can make a pendulum and have neutral density filters, so I went ahead an made up the experiment, using some scrap wire and a few optical-mount bases for the pendulum. And sure enough, with the right combination of filters (I think I ended up with ND=0.3 or 0.4) I was able to easily see the effect — the pendulum’s oscillation got closer at one end and further away at the other, and it reversed itself when I switched and darkened the other eye.

I wanted to show this off, so to make things a little easier I salvaged a damaged pair of sunglasses and popped one of the lenses shades out. (Not really lenses are they? OK, pedantic man says they are, albeit with an infinite focal length) That covers the eye better than the ND filter, which only has a 1″ diameter. I can use either the free eyepiece or the glasses. The biggest problem is getting people to look with both eyes — their natural tendency is to shut one eye and only look through the shade.