MCQ
$A$ monochromatic light source of wavelength $\lambda$ is placed at $S$. Three slits $S_1, S_2$ and $S_3$ are equidistant from the source $S$ and the point $P$ on the screen. $S_1P - S_2P = \lambda /6$ and $S_1P - S_3P = 2\lambda /3$. If $I$ be the intensity at $P$ when only one slit is open, the intensity at $P$ when all the three slits are open is
  • $3\, I$
  • B
    $5\, I$
  • C
    $8\, I$
  • D
    zero

Answer

Correct option: A.
$3\, I$
a
Given, $S_{1} P-S_{2} P=\frac{\lambda}{6}$

Hence, phase difference between $S_{1}$ and $S_{2}, \phi=\frac{\lambda}{6} \times \frac{2 \pi}{\lambda}=\frac{\pi}{3}$

So intensity between $S_{1}$ and $S_{2}$ is,

$I^{\prime}=I \cos ^{2}\left(\frac{\pi}{6}\right)^{2}=\frac{3 I}{4}$ $............(i)$

Given, $S_{1} P-S_{3} P=\frac{2 \lambda}{3}$

Hence, phase difference between $S_{1}$ and $S_{3}, \phi=\frac{2 \lambda}{3} \times \frac{2 \pi}{\lambda}=\frac{4 \pi}{3}$

So intensity between $S_{1}$ and $S_{2}$ is,

$I^{\prime \prime}=I \cos ^{2}\left(\frac{2 \pi}{3}\right)^{2}=\frac{I}{4} \quad \ldots \ldots \ldots(i i)$

The intensity at P when all the three slits are open is, $I \frac{I^{\prime}}{I^{\prime \prime}}=3 I$

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Question: Choose the correct answer from the options given below

A particle starts with $S.H.M.$ from the mean position as shown in the figure. Its amplitude is $A$ and its time period is $T$. At one time, its speed is half that of the maximum speed. What is this displacement?