  We consider two dimensional and three dimensional semi-infinite tubes made of
"Lambertian" material, so that the distribution of the direction of a reflected
light ray has the density proportional to the cosine of the angle with the
normal vector. If the light source is far away from the opening of the tube
then the exiting rays are (approximately) collimated in two dimensions but are
not collimated in three dimensions.
  An observer looking into the three dimensional tube will see "infinitely
bright" spot at the center of vision. In other words, in three dimensions, the
light brightness grows to infinity near the center as the light source moves
away.
