# Young double slit experiment

Can we say that the central maximum is the brightest fringe? If so, to find the width of the central maximum, why can't we say that it is twice the width of the brightest fringe (due to 2 times $\theta$)?

• You seem to be confusing the interference fringes with the diffraction envelope which modulates the intensity of the interference fringes? Jun 21 '18 at 5:33

$$I = 4I_ocos^2(\delta/2)$$ where δ is the phase difference between the two light rays (emanating from the two individual slits) at a given point on a screen due to path difference, initial phase difference, or optical phase difference. Therefore intensity will be maximum at points where δ is an even integral multiple of π (0,2π,4π...). These are points of constructive interference. Thus the central maximum is one but not the only point of maximum intensity.