CBSE 2026 · Region 3 · Set 3 · Q21 · 2 marks
In Young's double slit experiment, the central maximum is bright in the interference pattern obtained on a screen. What will happen when :(a)light waves emitted out of slits $\displaystyle \mathrm{S}_{1}$ and $\displaystyle \mathrm{S}_{2}$ have an initial phase difference of $\displaystyle \pi$ radian ?(b)one of the slits is closed ? Justify your answers.
In Young's double slit experiment, the central maximum is bright in the interference pattern obtained on a screen. What will happen when :
(a)
light waves emitted out of slits $\displaystyle \mathrm{S}_{1}$ and $\displaystyle \mathrm{S}_{2}$ have an initial phase difference of $\displaystyle \pi$ radian ?
(b)
one of the slits is closed ? Justify your answers.
Marking-scheme solution
(a)
The central maximum will be shifted by $\displaystyle \pi$ radian / the centre will become dark.
The condition for constructive interference is: phase difference between two waves $\displaystyle =2 n \pi$. For an initial phase difference of $\displaystyle \pi$ radian, phase difference $\displaystyle =2 n \pi \pm \pi=(2 n \pm 1) \pi$
For the central maximum, $\displaystyle n=0$, hence phase difference $\displaystyle = \pm \pi$ radian. (Alternatively: the path difference between the interfering waves will become $\displaystyle \lambda / 2$.)
(b)
Interference pattern disappears / uniform illumination will be observed. Two coherent sources are required for interference to take place.
Wave OpticsInterference of Light Waves and Young’s ExperimentUnderstandvery_short_answermedium
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CBSE Class 12 Physics past-paper question from the 2026board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.