Answer: Option A
:
A
$K_{\alpha }$x-ray emission takes place when an electron jumps from L-shell to fill a vacancy created in K-shell.
Now $E_K$is the energy of the atom when vacancy is in K-Shell, and $E_L$Is the energy of the atom when vacancy is created in L-Shell. Also, $E_K$ >$E_L$So, when vacancy shifts from K to L, according to our agrument, the atomic energy decreases. How much ?
$E_K-E_L$.
This difference results into X-ray photon.
Hence connecting the dots we can conclude that ; The energy difference between K and L levels is the energy of the photon.
Mathematically;
$E_K-E_L=E_{photon}$
Now,
$E_K-E_L=\frac{hc}{\lambda }$
Which gives after putting in the values;
$E_K-E_L=\frac{1242eVnm}{0.0021nm}$
Final calculations leads us to our answer;
$E_K-E_L=591428eV$
Or,
$E_K-E_L=0.59MeV$