A) 20
B) 60
C) 40
D) 80
Correct Answer: C
Solution :
The wavelength of X-ray lines is given by Rydberg Formula \[\frac{1}{\lambda }=R{{Z}^{2}}\,\left( \frac{1}{n_{1}^{2}}-\frac{1}{n_{2}^{2}} \right)\] For \[{{K}_{\alpha }}\] line, \[{{n}_{1}}=1\] and \[{{n}_{2}}=2\] \[\therefore \,\,\frac{1}{\lambda }=R{{Z}^{2}}\left( \frac{3}{4} \right)\] \[\Rightarrow \,Z={{\left( \frac{4}{3R\lambda } \right)}^{1/\,2}}\] \[={{\left[ \frac{4}{3\,(1.097\times {{10}^{7}}{{m}^{-1}})\,(0.76\times {{10}^{-10}}m)} \right]}^{1/\,2}}\]= 39.99 » 40
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