A) \[2.945\times {{10}^{-7}}m\]
B) \[3.945\times {{10}^{-7}}m\]
C) \[4.95\times {{10}^{-7}}m\]
D) \[1.945\times {{10}^{-7}}m\]
Correct Answer: A
Solution :
If thin film appears dark \[2\mu t\cos r=n\lambda \] For normal incidence\[r={{0}^{o}}\] \[\Rightarrow \] \[2\mu t=n\lambda \,\,\,\,\Rightarrow \,\,\,\,t=\frac{n\lambda }{2\mu }\] \[\Rightarrow \] \[{{t}_{\min }}=\frac{\lambda }{2\mu }=\frac{5890\times {{10}^{-10}}}{2\times 1}\] \[=2.945\times {{10}^{-7}}m\]
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