Thickness \({t}={n} \lambda\)
\({So}, {n} \lambda_{{vac}}=({n}+1) \lambda_{{air}}\)
\({n} \lambda=({n}+1) \frac{\lambda}{\mu_{{air}}}\)
\(n=\frac{1}{\mu_{\text {air }}-1}=\frac{10^{4}}{3}\)
\({t}={n} \lambda\)
\(=\frac{10^{4}}{3} \times 6000\, \stackrel{\circ}{{A}}\)
\(=2\, {mm}\)