Light waves enter the lens of the eye and project an upside-down image on the retina, which our brains turn right side up again. If you were fitted with glasses (me) that turned images upside down, after a while your brain would set things right for you, turning the images you see right side up again. When you took off the glasses, you would once again see the world upside down, until your brain readjusted and turned things right side up again.
Huh ! what i must try this someday - i wonder how long it takes to turn things up side down around again ? weird !