X-rays, because of their higher energies, have the property of penetrating matter which is opaque to visible light.

This does not mean that all substances are equally radiotranslucent; if this were so, radiography would be impossible.

An average bean from small diagnostic unit generated at 60 kV will penetrate a millimeter-thick sheath of aluminum but will be absorbed by the same thickness of lead.

The radio-opacity of any substance depends upon the the hardness or penetrating ability of the incident beam and also upon the atomic number, density and thickness of the material being irradiated.