Transparent playing cards would surely seem to be somewhat of an outright impossibility- How would one play with a deck that would allow light to pass through it, & yet conceal what cards you are holding from your opponents?
However, such cards are made, and the are perfectly playable. However, it can be said that the makers almost always "cheat" when designing the cards. Firstly, to make them transparent, they ay are printed on thin sheets of plastic.
the cards overlaid on text , showing their transparent nature
As you may observe from the above image, only a certan portion of the cards are transparent, the rest covered with a form of design which is opaque. It is on this design where the face side of the cards are printed on. In this case, the maker has decided to only print the indexes of the cards and nothing else.
a closeup of the above. Note the ornamented borders and centerpiece.
the cards held up to strong light
the same, held up the other way. Note the opaque ovals, on which the indexes are printed
However, at least in this instance, the maker has decided, presumedly for reasons of economy, not to make the whole of the back design opaque, but only the small oval on which the indexes are printed. This causes the little ovals on which the indexes are printed to stand up when the cards are held up to strong light.