Just to complicate things a bit....
I'm not sure that's true, actually. The PHB says "a transparent wall of magical force", which is considerably different from glass. Is glass considered "solid" even to an effect which is a burst of light? How about a canvas tent wall vs. that light? What about a burst of sound, then?
If this were 3E, I'd probably say "well, the rules say that magic light acts in this weird specific way". In 4E, I'm more inclined to apply common sense about the type of obstruction vs. the effect under consideration (sort of like the rules on page 66 of the DMG about deciding that a gauzy curtain might have a vulerability to fire).