The postcards will allow viewers to access footage on their computers of the character Merlin performing a magic trick from within a spell book.
Using "augmented reality" technology created by Magic Symbol from Inition, the spell book "magically" appears on your computer screen when a special symbol on the postcard is held up to a webcam.
'Merlin', produced by Shine for BBC Wales, aims to update the story of the infamous sorcerer of Arthurian legend for a new audience and stars newcomers Colin Morgan, Bradley James, Angel Coulby and Katie McGrath alongside Anthony Head and Richard Wilson.
Along with TV, radio and cinema trails, bus sides and digital media, the "magical" activity will take place over three consecutive Saturdays leading up to the first episode of 'Merlin'.
The magic eye content will also be available on the 'Merlin' website, which hosts hidden content that can be released by using your mouse to trace secret combinations, magic tricks, character profiles and a cinema trail.
Naomi Gibney, head of marketing and pictures at BBC Vision, said: "The Merlin campaign uses this new augmented reality technology in a way that will have enormous appeal for the show's audience.
"By targeting them in non-BBC outlets we are also able to reach a much wider potential audience who might not otherwise have heard about Merlin. There is a lot more material on the website but this is a really compelling calling card."