Originally Posted by
SRD
From Shadow Conjuration...
Shadow conjurations are actually one-fifth (20%) as strong as the real things, though creatures who believe the shadow conjurations to be real are affected by them at full strength.
Any creature that interacts with the conjured object, force, or creature can make a Will save to recognize its true nature.
Spells that deal damage have normal effects unless the affected creature succeeds on a Will save. Each disbelieving creature takes only one-fifth (20%) damage from the attack. If the disbelieved attack has a special effect other than damage, that effect is only 20% likely to occur. Regardless of the result of the save to disbelieve, an affected creature is also allowed any save that the spell being simulated allows, but the save DC is set according to shadow conjuration’s level (4th) rather than the spell’s normal level. In addition, any effect created by shadow conjuration allows spell resistance, even if the spell it is simulating does not. Shadow objects or substances have normal effects except against those who disbelieve them.
Against disbelievers, they are 20% likely to work.