Two light rays, initially in phase and with a wavelength of 510 nm, go through different paths by reflecting from various mirrors shown in the figure below. (Such reflection does not itself produce a phase shift.)

(a) What least value of d will put the rays exactly out of phase when they emerge from the region? (Ignore the apparent slight tilt of the ray 2.)
answer in nm
(b) Repeat the question assuming that the entire apparatus is immersed in a protein solution with an index of refraction of 1.11.
answer in nm