Knowledge

Facts, science and history of optical illusions.
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Parallax Effect

The history of optical illusions in art

How do optical illusions work?

Ghost in a Tree

Source/credit: Liz Steward

Believe what you want, but this is a good example of pareidolia.
The gaps in the tree behind this odd couple look like a ghost. The eeriness of seeing a ghost in a tree is completed by the greenish hue of the old photo.

More optical illusions:

Pipe Cleaner Arms

Paperthin Building

Too many legs

Floating helmet and wheel

Lake View Garden

Dog jumping over Manhattan

Flying through the woods

Parking ramp

Yard Ghost

Ghosts

Fried Chicken Tenders

Dogs