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Answer
(1) Constructive interference (brightness) : There is constructive interference at a point and the brightness or intensity is maximum there, if the two waves of light of the same frequency arrive at the point in phase, i.e., with a phase difference of zero or an integral multiple of $2 \pi$ radians.
A phase difference of $2 \pi$ radians corresponds to a path difference $\lambda$ where $\lambda$ is the wavelength of light. Since
$\frac{\text { phase difference }}{2 x}-\frac{\text { path difference }}{\lambda}$.
for constructive interference with maximum intensity of light, phase difference $=0,2 \pi, 4 \pi$, $6 \pi$... rad
$=n(2 \pi) rad$
or path difference $=0, \lambda, 2 \lambda, 3 \lambda \ldots$, etc.
$=n \lambda$
where $n=0,1,2,3, \ldots$, etc.
(2) Destructive interference (darkness) : There is destructive interference at a point and the point is the darkest, i.e. the intensity of light is minimum, if the two waves of light of the same frequency and intensity arrive at the point in opposite phase, i.e., with a phase difference of an odd-integral multiple of $\pi$ radians. A phase difference $2 \pi$ radians corresponds to a path difference $\lambda$ where $\lambda$ is the wavelength of light.
$\therefore$ For destructive interference with minimum intensity of light, phase difference $=\pi, 3 \pi, 5 \pi$, ... rad
$=(2 m-1) \pi rad$
or path difference $=\lambda / 2,3 \lambda / 2,5 \lambda / 2$, e., etc.
$
=(2 m-1) \frac{\lambda}{2}
$
where $m=1,2,3, \ldots$ etc.
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