How many purple dots do you see in the image above? Are they all the same hue? This new optical illusion, developed by biomedical optics engineer Hinnerk Schulz-Hildebrandt, is more revealing than you ...

Add Yahoo as a preferred source to see more of our stories on Google. Don't stare too long. Spoiler: It's a trick question. They look kind of purple, but they're all technically the same blue. But ...

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