Using Blender 2.70 and the BI renderer:
This is the scene:
There’s a tube and a spot light at the other end of the tube (currently selected).
Render result:
(Freestyle activated for better illustration)
Question: The light does NOT light ANY point on the outside of the tube. Though it produces specular highlight on the outside. Why?
There is no other light, and the tube is not transparent.
File:
Test 131.zip (78.1 KB)
Most likely due to sloppy modeling ;). Add default cylinder and use solidify modifier - you’ll be faster at the correct result.
Edit: on a second look i take modeling part back: i might have had wrong point of view or day ;). New cylinder still helps…
The light is set to “No Shadow”, therefore it simply shines through solid objects.
I’ve removed the solidify modifier to avoid someone telling me it’s the solidify modifier and I should apply it. 
Anyway, there’s not much to model sloppy, and it’s only an example. Normals are correct, too. (My original model only consists only of two vertices, plus a screw and a solidify modifier, but for simplification, I’ve posted it as is. Anyway, it doesn’t make a difference.)
Oh…no shadow…ähem… ok, I see. It was an old (very old) file that I opened, so I wasn’t aware of this anymore.
You’re absolutely right! Thanks Ikari!
Newly added Cylinder with the same material does not shine through.
It’s easy to add one.
