Shadow of transparent object with normal map

I want to create car’s headlight and for this I’ve created simple mesh object from cube and set ray transparency and mirroring. I’ve also set normal map to this object to make my glass grooved.
Transparency, reflection and refraction work perfect, but there is some troubles with object’s shadow. It looks like as from object without normal map. Using displacement instead of normal map solves this problem, but in this case object should have very high subdivison level.
What could be done with this?

In attachment you can find test blend file and rendered image,



headlight.blend (154 KB)

If I understand it correctly, normal maps don’t bend light, they just affect the surface of the object. I’d say that if you are modeling just a car light, then use a displacement map, because it shouldn’t increase the render times too much. Else, if you are modeling a whole car, the shadow pattern from the light might not show up anyway, so if you are very concerned about the polygon count, use a normal map.