The voltage applied to the Coolidge X-ray tube is increased by $25\%$. As a result, the short wave limit of continuous X-ray spectrum shifts by $\mathrm{\Delta }\mathrm{\lambda }$. The initial voltage applied to the tube is

• A

  $\frac{\mathrm{hc}}{4\mathrm{e\Delta \lambda }}$
• Correct answer
  B

  $\frac{\mathrm{hc}}{5\mathrm{e\Delta \lambda }}$
• C

  $\frac{4\mathrm{hc}}{5\mathrm{e\Delta \lambda }}$
• D

  $\frac{5\mathrm{hc}}{4\mathrm{e\Delta \lambda }}$

Hence, the correct answer is option: 'B'.