the reason why our eyes can detect light is because of the 5 million cones located in the retina of the eye. we see optical ullisons because of binocular vision. our eyes actually invert together to focus on one object at a time. this creates an optical illusion as if an object is there. for example, look at a horizon in the distance, it will appear that everything eventually gets smaller because of binocular vision. there Is another specific term for this, however, I do not know of it off hand as of right now.