Some people do not understand the concept of an optical illusion.

Perhaps, after all, the man was nothing more than an optical illusion.

The sun coming up and going down is one of the world's great optical illusions.

Not just an optical illusion, but a real face looking back.

What he could see was wrong, like an optical illusion.

It was, of course, no more than an optical illusion.

Some have put this down to simply being an optical illusion.

Her eyes suggested two further things, but either or both could have been an optical illusion.

There is an optical illusion about every person we meet.

This was almost certainly the special type of optical illusion which can take place even though no one is actually looking at it.