The wavelength of the most energetic $X$-rays emitted when a metal target is bombarded by $40 \ keV$ electrons is approximately .............. $\mathring{A}$.

An $X$-ray tube operates at $50 \ kV$. It converts $1\%$ of the energy into $X$-rays. If the amount of heat produced is $495 \ W$, then the number of electrons striking the target per second is:

An $X$-ray tube operates at $50 \, kV$ and $20 \, mA$. The target material has a mass of $1.0 \, kg$ and a specific heat capacity of $495 \, J \cdot kg^{-1} \cdot ^\circ C^{-1}$. One percent of the applied electrical power is converted into $X$-rays,and the remaining energy is used to heat the target. Then:
$(1)$ $A$ suitable target material should have a high melting point.
$(2)$ $A$ suitable target material should have low thermal conductivity.
$(3)$ The rate of rise of temperature of the target is $2 \, ^\circ C/s$.
$(4)$ The minimum wavelength of the emitted $X$-rays is $0.25 \, \mathring{A}$.

$A$ potential difference of $20 \, kV$ is applied across an $X-ray$ tube. The minimum wavelength of $X-rays$ generated (in $\mathring{A}$) is

Hard $X$-rays produced from a Coolidge tube are due to .........

When $X$-rays are incident on an object in a dark room,they will appear as ........