When I place a slab it allows sunlight through when it is a full slab block it is normal.And I searched this and did not see a copy so please don't bring it up
MCPE-1726
Slabs allow light
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This is based on the game mechanics. Slabs are considered as transparent (because they don't fill the whole block).
Read on here http://www.minecraftwiki.net/wiki/Opacity