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In Young’s double slit experiment as shown in figure, interference of light waves were observed on the screen. Thomas Young made two pinholes S₁ and S₂ (very close to each other) on an opaque screen in front of parent source S

S₁ and S₂ behaved as coherent sources, and produced interference patten on the screen which has alternate bright fringe and dark fringes. This experiment proved Huygen’s wave theory of Light.

In YDSE as shown, the path difference SS₁ − SS₂ = \(\frac{λ}{3}\) This causes the shift in position of central bright point. The new position of central bright point is

\(-\frac{1}{3}\frac{λD}{d}\)

We know path difference \(S_2 P-S_1 P=\frac{y_n d}{D}+\frac{\lambda}{3} \)

$y_n$ is position of $n^{th}$ maxima.

For $n^{th}$ maxima , path difference = nλ

∴  \(\frac{y_n d}{D}+\frac{\lambda}{3}=n \lambda \)

\(\frac{y_n d}{D}=n \lambda-\frac{\lambda}{3} \)

For central bright fringe n = 0

∴ \( \frac{y_0 d}{D}=-\frac{\lambda}{3} \)

\( y_0=-\frac{\lambda D}{3 d} \)

$\text{ So the central maxima shifts by } -\frac{1}{3}\frac{λD}{d}$