Chapter 24 - Physical Optics: The Wave of Nature of Light - Learning Path Questions and Exercises - Exercises - Page 841: 19

$54.3nm$

$n_{1}=1.38$ $v_{b}=400nm$, $v_{r}=700nm$ ${t}_{min}=\frac{v}{4n_{1}}$ The difference in the minimum film thickness required for maximum transmission of blue light and red light = $\frac{700nm}{4\times1.38}-\frac{400nm}{4\times1.38}=54.3nm$