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.
> 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
No, any wave has a wavelength. You can add sin(3x) to sin(2x) and the resulting wave is a perfect fifth. Its wavelength is determined by its components; since sin(2x) has a wavelength of π and sin(3x) has one of 2π/3, the combined wave will have one of 2π.
The difference is that sin(2x) and sin(3x) are both sine waves, while their sum is not. There is no such thing as a pure tone of two merged frequencies, but there are many possible waves at any given frequency that aren't pure tones.