Fill in the blanks

Question

In double slit interference, if the optical path at point P isδ=1.5λ, then the fringe on point P of the screen is _____________, if the optical path at point Pis δ=5λ, then the fringe on point of the screen is_________.

 

Answer 1

The answers are "dark" and "light" respectively.  When the optical paths of the two photons differ by half a wavelength (nλ + λ/2) at point P, the photons from each slit will have opposite phases and will destructively interfere.  When the optical paths at P differ by a multiple of 1 wavelength (nλ), then the photons will be in phase and will constructively interfere.