In optical illusion mind send signal whatever it interpret at first look mind create an image by its entire surrounding image. Some time surround image background prevents our mind to accept truth.
An optical illusion (also called a visual illusion) is an illusion caused by the visual system and characterized by visually perceived images that differ from objective reality. The information gathered by the eye is processed in the brain to give a perceptive that does not tally with a physical measurement of the stimulus source.
|Block a and b appear different color but they are same as in next image is proof|
Image Source : https://www.wikipedia.org/