A beam of light consisting of two wavelengths 7000 o A and 5500 o A...

The given data is

${\lambda}_{1} = 7000\overset{o}{A}$

${\lambda}_{2} = 5500\overset{o}{A}$

$d = 2 . 5 \times {10}^{- 3}m$

$D = 1 . 5m$

The path difference is given by

$n{\lambda}_{1} = m{\lambda}_{2}$

$7n = 5 . 5m$

$⇒ 14n = 11m ⇒ n = 11$ and $m = 14$

The formula for the distance of a bright fringe is

$y = \dfrac{n{\lambda}_{1}D}{d}$

$⇒ y = \dfrac{11 \times 7 \times {10}^{- 7} \times 1 . 5}{2 . 5 \times {10}^{- 3}}$

$= 46 . 2 \times {10}^{- 4} = 462 \times {10}^{- 5}$

It is given that

$n \times {10}^{- 5} = 462 \times {10}^{- 5} ⇒ n = 462$