Sorry for the long quiet period, but I have been busy in the world of work.
Recently there was some fuss about the picture of the dress that different people perceive in different colours. I am sure you have seen it. Admittedly this has nothing to do with the connection of mathematics and physics, the topic of this blog. Still I thought it was pretty cool.
Here is how you can see both interpretations. The optical illusion is due to your brain performing a white balance operation by guessing at the lighting conditions. In one case your brain thinks the picture has been taken in yellow light, in the other case in blue light. If you want to switch the colour you see, you need to give your brain some more information about the surrounding. See the two pictures below:
The picture of the dress has another interesting property. The pictures of the stripes are inverses of each other. I saw a yellow and white dress (at first). Inverting the picture to its colour negative gives a white and yellow dress.