For constructive interference to occur, the difference between the two paths must be an integer multiple of the wavelength of the light $($as is true in any interference problem$),$ i$.$e$.$ the general criterion for constructive interference is

mm$,$constructive$=2$dmm$,$constructive$=2$d$,$

where$$mm$$is a positive integer. This is usually stated in the slightly more explicit form

m$,$constructive$=2$dmm$,$constructive$=2$dm$.$

Given the thickness of the film, dd$,$ what is the longest wavelength that can exhibit constructive interference?