• # question_answer When a beam of accelerated electrons hits a target a continuous X-ray spectrum is emitted from the target. Which one of the following wavelength is absent in the X-ray spectrum if the X-ray tube is operating at 40,000 volt? A) $1.5\overset{\text{o}}{\mathop{\text{A}}}\,$                     B) $~0.5\text{ }\overset{\text{o}}{\mathop{\text{A}}}\,$C) $0.25\overset{\text{o}}{\mathop{\text{A}}}\,$                   D) $1.0\overset{\text{o}}{\mathop{\text{A}}}\,$

Cut off wave length $={{\lambda }_{\min }}=\frac{hc}{eV}=\frac{12375}{40000}\overset{\text{o}}{\mathop{\text{A}}}\,=0.31\overset{\text{o}}{\mathop{\text{A}}}\,$ Hence, wavelength of $0.25\text{ }\overset{\text{o}}{\mathop{\text{A}}}\,$must be absent in the X-rays given out by the tube. Hence, the correction option is [c].