Chapter 24 - Physical Optics: The Wave of Nature of Light - Learning Path Questions and Exercises - Exercises - Page 841: 18

$91.96nm$

Refractive index of thin film = 1.4 , Refractive index of lens = 1.6 , Refractive index of air = 1. Wavelength of yellow-orange light = 515nm/ Hence, $t_{min}=\frac{v}{4n_{1}}=\frac{515\times10^{-9}}{4\times 1.4}=91.96nm$