In Coolidge X-ray tube (Modern X-ray tube) how will you vary the penetrating power of emitted X-rays?

What do you mean by minimum wavelength limit of X-rays? If a Coolidge X-ray tube is operated at $V$ volt then prove that the shortest wavelength of the X-rays emitted will be given by

${\lambda }_{min}=\frac{1.2375\times {10}^{-6}}{V}\mathrm{m}$.