This bizarre optical illusion has divided the internet as social media users can't agree whether the dots are purple or blue. And another joked: 'How does the purple dot know to change to where my eyes are looking?' This makes the other dots appear blue when we are not focusing on them. He writes: 'A pattern of purple objects on a blueish background appears only purple where the viewer looks directly at it. By placing purple dots on a similar blue background, our brains can only generate the correct purple colour for dots at the very centre of our attention.