Question

$n$ th bright fringe if red light $({\lambda }_1=7500\mathring{A}$ ) coincides with $(n+1)$ th bright fringe of green light $\left({\lambda }_2=6000\mathring{A}\right)$. The value of $n$, is

• A. 4
• B. 5
• C. 3
• D. 2

Answer 1

Answer: (A)

As the two bright fringes coincide
$\therefore n{\lambda }_1=(n+1){\lambda }_2$
$\frac{n+1}{n}=\frac{{\lambda }_1}{{\lambda }_2}$
$=\frac{7500}{6000}=\frac{5}{4}$
$1+\frac{1}{n}=\frac{5}{4}$
$\Rightarrow n=4$