Question

Find the location of third bright fringe from center of the screen

• A. $8.74\times {10}^{-4}m$
• B. $6.74\times {10}^{-4}mm$
• C. $5\times {10}^{-4}mm$
• D. $8.74\times {10}^{-7}m$

Answer 1

The correct option is A $8.74\times {10}^{-4}m$

Angle subtended between the Ist minima and central maxima is half minute. i-e, $\theta =(\frac{1}{2}{)}^{\prime }=\frac{1}{2\times 60}\frac{\pi }{180}=.000145degrees$ ($1^o={60}^{\prime }$)

Now for Ist minima, $d\theta =\frac{\lambda }{2}⟹d=3439.5\lambda$

For 3rd maxima, $y=\frac{3\lambda D}{d}=\frac{3\lambda \times 1}{3439.5\lambda }$

$⟹y=0.874mm$