Question

Mourning doves have a small patch of iridescent feathers. The color is produced by a 330-nm-thick layer of keratin (n=1.56)(n=1.56) with air on both sides that is found around the edge of the feather barbules. For what wavelength or wavelengths would this structure produce constructive interference?

-this question has been asked twice already but has gotten two different answers, I am trying to figure out what the correct answer is..

Answer 1