The middle of the first-order maximum, adjacent to the central bright fringe in a double-slit experiment, corresponds to a point where the optical path length difference from the two slits is equal to:
c) one wavelength.
d) 1/2 wavelength.
e) 1/4 wavelength.
The bright spots are as a result of constructive interference, which occurs when the path length differs by multiples of one wavelength. The first order maximum varies by exactly one wavelength.