Herman Grid Illusion
What do you see?
When you look at the grid, the white dots and the center of each ‘corridor’ seem to shift between white and gray. When you focused your attention on a specific dot, it is obvious that it is white. But as soon as your attention is shifted away, the dot shifts to a gray color.
So why do people see gray where there should be white? Why do we see something so different from reality?
This phenomena demonstrates a very important principle of perception: we don’t always see what’s really there.
Ames Room Youtube video The effect is used in The Lord of the Rings trilogy. Note the early scenes in The Fellowship of the Ring where the effect is y used to make Gandalf appear larger than the hobbits.
Ponzo Illusion (Linear Perspective)
The reason the top horizontal line looks longer is because we interpret the scene using linear perspective. Since the vertical parallel lines seem to grow closer as they move further away, we interpret the top line as being further off in the distance. An object in the distance would need to be longer in order for it to appear the same size as a near object, so the top “far” line is seen as being longer than the bottom “near” line, even though they are the same size.
Kanizsa Triangle (Gestalt prinicple-Law of Closure)
Gestalt psychologists use this illusion to describe the law of closure. According to this principle, objects that are grouped together tend to be seen as being part of a whole. We tend to ignore gaps and perceive the contour lines in order to make the image appear as a cohesive whole.
figure-ground relationship deals with how we distinguish
what objects in our vision do we want to examine (figure) and what parts are
just background (ground).
goodpsychology 
