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People say this optical illusion with 2 dots is blowing their minds — what do you see?

INSIDER logoINSIDER 1/11/2018 (Janaki Jitchotvisut)

a close up of a lamp: Optical illusions © Provided by Business Insider Inc Optical illusions

  • -This optical illusion is stumping the internet. 
  • -People can't tell how many dots there are and which direction they're moving. 
  • -You can thank the stroboscopic effect for making this and many other optical illusions possible.

This optical illusion will have you questioning what's right in front of you. 

This illusion posted on Reddit by user d023n involves flashing dots that strobe onscreen — the only question is, how many? 

5 Hertz

Are we looking at four dots flashing? Or are we seeing two dots moving around? To me, it looks like two dots bouncing up and down like rubber balls, but you might see something completely different.

Do you see two dots moving from side to side? Perhaps you see them moving around clockwise — or counter-clockwise. Or maybe you see four dots. In any case, the caption tells us that the lights are flashing at a frequency of five Hertz.

Further down the thread, d023n played with this GIF so viewers could analyze it at different speeds. 

Here's one at two Hertz, where you can see that there are two dots, as well as all the movement possibilities if you just stare at it for awhile.

2 Hertz

Here's a sped-up version at 10 Hertz, which is way too frantic to wrap your brain around.

10 Hertz

So, what are we really seeing?

If you want to take a peek behind the curtain like I always do, read on. 

Have you ever seen train wheels (or anything else spinning) that suddenly start to look like they're standing still? Or even spinning backward, although you can see the train is still moving forward?

That's a phenomenon called the stroboscopic effect, and here's an extremely helpful 4-minute video by the BBC to demonstrate exactly how it works. Basically, when there's a strobing motion of light, it can trick your eye into thinking something is moving backward or not be moving at all when it may just be moving normally. 

Just like your TV or computer screen, human eyes have a "frame rate." It's called the critical flicker frequency, and it's what determines the frequency at which we need to see images for them to look smooth instead of flickering. The strobe effect can mess with that flicker frequency, resulting in the trick of the eye. 

Related: The Best Optical Illusions of 2017

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