Question

In the adjacent diagram, CP represents a wavefront and AO and iBP, the corresponding two rays. Find the condition of $\theta$ for constructive interference at P between the ray BP and reflected ray OP

• A. $cos \, \theta=\frac{3 \lambda}{2d}$
• B. $cos \, \theta=\frac{\lambda}{4d}$
• C. $sec \theta-cos \, \theta=\frac{\lambda}{d}$
• D. $sec \theta-cos \, \theta=\frac{4 \lambda}{d}$

Answer 1

Answer: (B)

$cos \, \theta=\frac{\lambda}{4d}$

Answer 2

PR=d
\therefore \hspace10mm PO=d \, sec \theta
and \hspace10mm CO=PO cos \, 2 \, \theta=d \, sec \, \theta cos \, 2 \, \theta
path difference between the two rays is,
$\, \, \, \, ? x=PO+OC=(d \, sec \theta+d \, sec \, \theta \, cos \, 2 \, \theta)$
phase difference between the two rays is
$? \phi =\pi$(one is reflected, while another is direct)
Therefore, condition for constructive interference should be
\hspace20mm ? x=\frac{\lambda}{2},\frac{3 \lambda}{2}...
or \hspace10mm d \, sec \, \theta (1+ cos \, 2 \, \theta)=\frac{\lambda}{2}
or \hspace15mm \big(\frac{d}{cos \theta}\big)(2 cos^2 \theta)=\frac{\lambda}{2}
or \hspace30mm cos \theta=\frac{\lambda}{4d}