The shortest wavelength emitted from an $X$-ray tube depends on which of the following?

$A$ metal block is exposed to beams of $X$-rays of different wavelengths. $X$-rays of which wavelength penetrate most?

For the production of characteristic $K_{\gamma}$ $X$-ray,the electron transition is:

$X$-rays are produced when ...

An $X$-ray tube has a short wavelength limit at $0.45 \mathring{A}$. The voltage of the tube is ........ $V$.

The wavelength of $L_{\alpha}$ line in $X$-ray spectrum of $_{78}Pt$ is $1.32 \ \mathring{A}$. The wavelength of $L_{\alpha}$ line in $X$-ray spectrum of another unknown element is $4.17 \ \mathring{A}$. If the screening constant for the $L_{\alpha}$ line is $7.4$,then the atomic number of the unknown element is: