A) \[25\]
B) \[0.25\]
C) \[0.25\times {{10}^{4}}\]
D) \[25\times {{10}^{4}}\]
Correct Answer: C
Solution :
Let n be the number of waves then \[n\lambda =l\] Given, \[l=1mm={{10}^{-3}}m,\] \[\lambda =4000\overset{\text{o}}{\mathop{\text{A}}}\,=4000\times {{10}^{-10}}m\] \[\therefore \] \[n=\frac{{{10}^{-3}}}{4000\times {{10}^{-10}}}=0.25\times {{10}^{4}}\]
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