Two wavelengths do it; one does not suffice. It's like a perfect fifth can not one note.
replyThe interference is a wavelength too. Maybe not pure but it is one. Afaik they cannot be interpreted as two separate wavelengths and then “brain combined” when the aperture (the retina) is so small.
replyI haven't heard of a wavelength of 2 frequencies merged. It is like saying what is the wavelength if you tune to 2 radio stations with 2 radios (assume silent transmition for simplicity). There are 2 wavelengths.
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