Question

Find the fringe width for the pattern obtained under given arrangement on the screen.

• A. $\frac{\lambda f}{2t}$
• B. $\frac{\lambda f}{t}$
• C. $\frac{tf}{\lambda }$
• D. $\frac{tf}{2\lambda }$

Answer 1

Answer: (B)

$\frac{\lambda f}{t}$

By lens equation, $\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$

$\frac{u}{v}-1=\frac{u}{f}$

$\frac{-1}{m}-1=\frac{u}{f}$

$|m|=\frac{f}{f+u}$

$d=\frac{ft}{f+u}$

$|v|=\frac{fu}{f+u}$

$w=\frac{D\lambda }{d}=\frac{\lambda (\frac{fu}{f+u}-f)}{\frac{ft}{f+u}}=\frac{f\lambda }{t}$