What does the expression 'to fool the eye' mean?

Tromp l'oeil or "fool the eye" is a painting device that gets the viewer to take a painted object or surface for an actual, physical object or surface. This could be an ornate chained magnifying glass that hangs next to a highly detailed painting of a barn. The viewer reaches for the magnifying glass only to realize that it is a tightly rendered painting of a magnifying glass and their fingers touch only cold wall.