The difference in the number of waves when yellow light propagates...

Thickness $t = n\lambda$

So, $n{\lambda}_{vac} = ( n + 1 ) {\lambda}_{air}$

$n\lambda = ( n + 1 ) \dfrac{\lambda}{{\mu}_{air}}$

$n = \dfrac{1}{{\mu}_{air} - 1} = \dfrac{{10}^{4}}{3}$

$t = n\lambda$

$= \dfrac{{10}^{4}}{3} \times 6000\overset{0}{A}$

$= 2mm$