Question: How do illusions work?

Michael Cook answered on 3 Jul 2012:

Good question!

Some illusions or magic are based on sleight of hand – that is, the person doing the trick are simply very talented at doing things quickly, or out of sight. They distract you with one hand, while the other hand pulls out a hidden card or hides away a ring.

If you mean the sort of illusions that people stare at on paper, lots of them take advantage of the way our eyes and brain have evolved to work. For instance, humans are very good at ‘filling in the gaps’ in an image where they don’t have all the information they need. This is because it helps us guess at what the best thing to do might be, which is especially useful for making quick decisions. Here’s a good example of that, where there appears to be a white triangle even though we can’t see the shape really: http://en.wikipedia.org/wiki/File:Kanizsa_triangle.svg

Evolution creates very specialised organisms. You can see lots of animals that have unique skills that let them do amazing things like running very fast, surviving extreme temperatures, and so on. The human brain is a bit like that too – very focused on certain things. Optical illusions take advantage of that!