How Does Thin-Film Interference Affect Light Reflection in Soap Films?

[Laceb04]

Light of wavelength 654 nm (in vacuum) is incident perpendicularly on a soap film (assume n = 1.32) suspended in the air. What are the two smallest nonzero film thicknesses (in nm) for which the reflected light undergoes constructive interference?
__________nm (smallest thickness)
__________nm (next smallest thickness)

Info: Thin-Film Interference

 

[Doc Al]

Please show what you've done so far and where you are stuck. (Hint: Consider the interference between light reflecting from the top and bottom surfaces of the soap film.)