Reducing 1 1/2" conduit to 1/2" liquid tight
I've only seen liquid tight with threaded connections, like this:



So basically you need to go from 1 1/2" to 1/2" threaded. 

There are reducing bushings, like this:



Finding 1 1/2" to 1/2" seems to be difficult (that's a big drop) -- but they do exist (I can find them in catalogs, just can't find an actual picture).

If you can get a 1" or 3/4" to 1/2" (much more likely) then you just also need a reducing bushing (or two) to go from 1 1/2" to that size:





Another way to make the connection is to use a 1/2" female adapter on the end of a short piece of PVC pipe:



Again, you'd have to use bushings to get from the 1 1/2" box to the 1/2" PVC pipe.



Be sure to use PVC cement to make the connections and keep it water-tight.