Why are we fooled by optical illusions?
Optical Illusions are things that make us see something that doesn't exist, make us see something in way that we shouldn't see it or otherwise trick our visual perception.
After Images, etc
Stare at the flag for 30 seconds. After
that, stare at a white wall, piece of paper or a white point on this webpage.
You should see an after image with the inverse colors of the original.
This happens becasue the rods and cones in yours eyes that percieve the
colors in the first image get tired. This is why if you glance at the
sun (and we do NOT recommend that you do) you have a bright spot in your
field of vision for a short amount of time.
Do you see the triangle in the image below?
There is no triangle... It's just three circles with little pieces taken
out. You just assume that there's a triangle.
If you take a picture of something that was a lot of variance in depth (like a hole in the ground), and look at it upside down, it will appear like it is popping out where it was popping in and vice versa.