Y and Z are two partially reflecting glass sheets, a distance l apart in air. White light falls normally on Y. The light emerging from Z is analysed by a spectrometer. A dark band is observed in the spectrometer at a wavelength. The distance l must be equal to:

• A. nλ/2
• B. (n + 1/2)λ/2
• C. (n + 1/4)λ/2
• D. (n + 1/2)λ