Optical: of or pertaining to sight or vision; visual
Illusion: something that deceives by producing a false or misleading impression or reality
Scientists believe optical illusions are possible because our brains are so good at recognizing patterns and seeing familiar objects. Our brains are quick to make whole images from separate pieces. Therefore our eyes and brains can be quick to see things which aren't really there.
A very simple optical illusion would be this one.
The different arrow points, make the 2nd line appear larger, which it is not.
Here's a picture of Albert Einstein.
Now walk away from the computer, turn around and look at it from afar. Who is it a picture of ?
And saving the best for last, I was actually introduced to the artwork of Julian Beever by my daughter who discovered his work on Tumbler.
These 3D sidewalk drawings are absolutely amazing! It's no wonder he has been nicknamed "The Pavement Picasso". Here is the link to his website, if you want to see more of his mind boggling art http://www.julianbeever.net/.
TTYL and make it a good day,