The diagram below shows two paths that incoming white light might follow when striking the surface of water with a thin
film of soap over it. Which of the following must be true
concerning the distance traveled by red light within the soap film
so that upon emerging along path B it destructively interferes
with red light that reflected off the soap along path A?
a) The distance travelled within the soap = 1 wavelength of red light
b) The distance travelled within the soap = 1/2 wavelength of red light
c) The distance travelled within the soap = 2 wavelength of red light
d) The distance travelled within the soap = 2 wavelength of blue light