I have a rapier which has been enchanted with electrical damage, level one. Part of the enchantment seems to be a glow - when I draw the sword I have light to see by.
I start out with bow in hand, cast a light spell to be able to see.
Switch to rapier when the baddies get close, the light goes out. The light effect of the sword, being apparently 180° reversed in phase to my light spell, causes dark and annuls my light spell.