Correct Answer - `0001`
Reflected ray from upper surface would would shift by `lambda//2` only while reflected from lower surface would not have any shift.
`2mut=n_(1)lambda_(1)=n_(2) lambda_(2) Rightarrow (n_(1)=n_(2)+1)`as there is no minima in between these two wavelengths
`(n+1)(512)=(n)(640)`
`n_(2)(640-512)=512`
`n_(2)=4`
So `2xx1.28t=(4)(640)`
`t=(4xx640)/(2xx1.28)=1000` nm=1mum