Why Is m Equal to 1 for Minimum Thickness in Thin Film Interference?

[math10e]

wavelengths-->calculating "m"

A thin film of turpentine (n=1.36) covers a lanthanum flint glass plate (n=1.80). What is the minimum thickness the film should have if it is to appear green (wavelength=560nm) when viewed from above in white light? (answer=206nm)


Use the equation 2t(n)=mλ



I know that it would be 2t(1.36)=1(560), but why is "m" 1? I know that there is a 180 degree phase change making the waves constructive...but does m=1 only because there needs to be some light emitted?

 

[Sourabh N]

because you have to find minimum thickness, thickness is directly proportional to m and m can only be an integer