Coherent, monochromatic light of wavelength 720 nm is incident on two closely-spaced, very narrow slits. The distance between the slits, the distance to the viewing screen behind the barrier, and location of the bright and dark spots have not been measured. We can conclude that light leaving the more distant slit which arrives at the 2nd dark spot relative to the central bright spot has traveled an extra distance of ________ relative to the closer slit.
a) 720 nm
b) 840 nm
c) 960 nm
d) 1080 nm
e) 1200 nm