The 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.

I 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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