Our brain wants to see one thing. Optical illusions are designed to show many things and confuse the brain.
Because that is what it is. It creates optical effects, makes us see things which are impossible or seem to be what they are not.
Rods and cones are things in your eye that help you to see black and white. Rods are more sensitive than cones. They're what makes optical illusions possible for our eyes along with perception. Rods are sensitive to only the brigtness of the light, and allow us to see in black and white. Cones provide color vision.
The question is too vague to be meaninful. Some optical illusions INVOLVE color, so for them I suppose the answer would be yes. Others don't, and the answer there might be either no or yes; it's difficult to say without more details.
Yes, optical microscopes can see cells.
Perception. Do be careful of your definition of the word "fact". Our perception is known to deceive us in many instances. These instances include optical illusions, and selective-seeing/hearing
balls
We see deep into space by using optical or radio telescopes, either from the Earth's surface, from satellites orbiting above us, or carried out into space on an unmanned spacecraft.
Despite many urban legends, internet rumors, stories from a friend of a friend, and so on - - there are no secret markings on US bills. Many of the claimed "mystery" symbols such as the reported "owl" on a $1 bill are nothing but optical illusions that disappear when viewed with a magnifying glass.
an optical illusion misleads us about the relative size or distant of an object by placing it in an unexpected situation
Refracted light
Its not so much "illusions" that psychologists like, but things that cause us to change our perceptions. This can be something as random as an ink blot or as concrete as a painting by M.C Escher. By asking thousands of people to explain what they see (or to be more precise, perceive) in these various illusions, psychologists have built up a sort of catalog of interpretations that allow them to get a better idea of how your mind works, what your dispositions are, what the state of your sub-conscious mind is, and a wide variety of other things that will help them to treat you that you may have been unwilling or unable to explain to them.
It is the reflection of the sun's light on the moon that makes it visible to us here on Earth.