Superman used X-ray vision to detect objects. Aside from a source of X-rays, what else would be needed for X-ray vision?

an absorber of x-ray

Explanation:

To make x-ray detection/vision work, you will need at least two items: a source of x-ray and absorbed or x-ray.  

The object you want to see itself doesn't have to be the source, but it has to absorb some of the rays instead. When doing a chest x-ray test, the medical employee will put your chest between absorber and source. The heart can absorb some of the rays so it will appear more white than lungs who made of air and won't absorb the rays.

0.11 m/s and 0.28 m/s should be correct