We're all fans of a good optical illusion - what else would make people millions of people freak out over the colour of a blue and black dress? But what actually causes our eyes to see things that aren't there? In this episode, Destin from Smarter Every Day explains the science behind optical illusions. But before we get into that, you need to watch at least the first minute to appreciate just how trippy these mind tricks can be, because - what the hell! Why is that island suddenly black and white?

So what's going on here? To find out, Destin goes to meet Greg, the creator of a viral optical illusion video, who creates incredibly complex images out of a simple spinning circuit board covered in red, blue, and green LED lights. And he takes his trusty Phantom slow-motion camera with him to find out what our eyes aren't seeing.

When the LED device starts spinning, our eyes see it as a complete image. But when Destin films it at a super-slow 5,500 frames per second, it becomes clear that what we're really looking at is mostly dead space and a few carefully selected LEDs.

So why does our brain turn that into something else? Why can't our eyes see what's really going on? According to Stuart Anstis, an expert on visual perception from the University of California, San Diego, it all comes down to something called 'persistence of vision'.

Our eyes have evolved sort of like a long-exposure camera, which means they take in a lot of light and have greater sensitivity, but they're also pretty slow to respond to new information.

As Anstis explains in the video above, it can take anywhere from 10 to 100 milliseconds for our brain to register light. And so our eyes average out what they see over a short period of time, to smooth out our vision for us.



That makes sense, but Destin takes things one step further and actually reverse engineers the process, to figure out at what point our eyes stop seeing a uniform image and start seeing a spinning column of light. And the results are just mind-blowing.

Watch above to see it for yourself, because our explanation does not do it justice. And next time you get mad at your human eyes for tricking you, just remember that if it wasn't for this engineering 'flaw' the world would look a whole lot less pretty. You're welcome.