Question

Two sources of light of wavelengths 2500 $\mathring{A}$ and 3500 $\mathring{A}$ are used in Young's double slit experiment simultaneously. Which orders of fringes of two wavelength patterns coincide?

• A. 3rd order of 1st source and 5th of the 2nd
• B. 7th order of 1st and 5th order of 2nd
• C. 5th order of 1st and 3rd order of 2nd
• D. 5th order of 1st and 7th order of 2nd

Answer 1

The correct option is B 7th order of 1st and 5th order of 2nd

Let $n^{th}$ fringe of 2500$\mathring{A}$ coincide with $(n-2{)}^{th}$ fringe of 3500 $\mathring{A}$.
$\therefore 3500(n-2)=2500\times n$
$1000n=7000$,

$n=7$
$\therefore$ 7th order fringe of 1 st source will coincide with 5th order fringe of 2nd source.