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The rope shown at an instant is carrying a wave travelling towards right, created by a source vibrating at a frequency $n$. Consider the following statements
$I.$ The speed of the wave is $4n \times ab$
$II.$ The medium at $a$ will be in the same phase as $d$ after $\frac{4}{{3n}}s$
$III.$ The phase difference between $b$ and $e$ is $\frac{{3\pi }}{2}$
Which of these statements are correct

$I, II$ and $ III$
$II$ only
$I$ and $III$
$III$ only
Solution
(c) Speed = $n\lambda $= $n(4ab) = 4n \times ab$ $\left( {{\rm{As}}\,ab = \frac{\lambda }{4}} \right)$
Path difference between $b$ and $e$ is $\frac{{3\lambda }}{4}$
So the phase difference =$\frac{{2\pi }}{\lambda } \times $Path difference
= $\frac{{2\pi }}{\lambda }.\frac{{3\lambda }}{4} = \frac{{3\pi }}{2}$