Question

A single lens is mounted in a tube. A parallel beam enters the tube and emerges out of the tube as a divergent beam. Can you say with certainty that there is a diverging lens in the tube?

Answer 1

Assuming that the tube is in the air and the lens is made of a material having the refractive index µ > 1, it can not be said with certainty that there is a diverging lens (concave lens) inside the tube. A converging lens (convex lens) having its focal point well inside the tube will also let this beam emerge out as a diverging beam.