Question

In an interference pattern the $(n+4{)}^{\text{th }}$ blue bright fringe and $n^{\text{th }}$ red bright fringe are formed at the same spot. If red and blue light have the wavelength of $7800\mathring{A}$ and $5200\mathring{A}$ then value of $n$ should be

Answer 1

Answer: 8

Fringes are formed at same spot means fringes coincide
$n_1{\lambda }_1=n_2{\lambda }_2$
$(n+4)5200\mathring{A}=(n)7800\mathring{A}$
$n=8$