$X$-rays will travel the minimum distance in which of the following materials?

Penetrating power of $X$-rays increases with an increase in

$X-$rays are in nature similar to

The energy of a photon of characteristic $X-$ rays from a Coolidge tube comes from

When a beam of accelerated electrons hits a target,a continuous $X$-ray spectrum is emitted from the target. Which of the following wavelengths is absent in the $X$-ray spectrum,if the $X$-ray tube is operating at $40,000 \ V$?

$A$ potential difference of $42,000 \ V$ is used in an $X-ray$ tube to accelerate electrons. The maximum frequency of the $X-radiations$ produced is ($1 \ eV = 1.6 \times 10^{-19} \ J$ and $h = 6.63 \times 10^{-34} \ J \cdot s$).