Q.The energy of a photon of characteristic X-ray from a Coolidge tube comes from :

X-rays, discovered by Wilhelm Roentgen in 1895, are highly penetrating electromagnetic waves of very short wavelength (about 0.1 Å to 100 Å) produced whenever fast-moving electrons strike certain target materials. They are generated in an evacuated X-ray tube: a heated tungsten filament emits electrons by thermionic emission, these are accelerated to high speed by a large applied voltage, and they strike a target (commonly tungsten or molybdenum, chosen for their high melting points) embedded in a tilted, cooled copper anode, allowing the resulting X-rays to escape sideways through the tube wall.

An X-ray spectrum -- intensity plotted against wavelength -- always consists of two superimposed parts. The continuous spectrum arises from Bremsstrahlung ('braking radiation'): as fast electrons are decelerated by target nuclei, they radiate away their lost kinetic energy as photons of a whole continuous range of energies, since different electrons happen to lose different amounts of energy in different encounters. The special extreme case, where an electron gives up its entire kinetic energy $eV$ (accelerating voltage $V$) to a single photon, sets the minimum wavelength (maximum frequency) of this continuous spectrum, given by the Duane-Hunt formula $\lambda_0=hc/eV=12400/V\ \text{\AA}$ -- a cut-off wavelength that depends only on the tube's accelerating voltage, not on the target material, exactly matching what classical theory could never explain but the photon picture explains directly. Superimposed on this continuous background is the characteristic spectrum: a handful of sharp peaks (such as the $K_\alpha$ and $K_\beta$ lines) at wavelengths unique to the target element, arising when an energetic bombarding electron knocks an inner K-shell electron out of a target atom and an outer-shell electron falls to fill the vacan …