A) \[8000\overset{\text{o}}{\mathop{\text{A}}}\,\]
B) \[7000\overset{\text{o}}{\mathop{\text{A}}}\,\]
C) \[1472\overset{\text{o}}{\mathop{\text{A}}}\,\]
D) \[2950\overset{\text{o}}{\mathop{\text{A}}}\,\]
Correct Answer: D
Solution :
Suppose the cut off wavelength is represented by\[{{\lambda }_{0}}\]. so, \[\frac{hc}{{{\lambda }_{0}}}=\]work function\[={{W}_{0}}\] \[\frac{hc}{{{\lambda }_{0}}}=4.2\times 1.6\times {{10}^{-19}}\] \[{{\lambda }_{0}}=\frac{6.6\times {{10}^{-34}}\times 3\times {{10}^{8}}}{4.2\times 1.6\times {{10}^{-19}}}\] \[=2946\times {{10}^{-10}}m\] \[\approx 2950\,\,\overset{\text{o}}{\mathop{\text{A}}}\,\]
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