Question

Two sources of light of wavelengths 2500 $\stackrel{o}{A}$ and 3500 $\stackrel{o}{A}$ are used in Young's double slit expt. 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 nth fringe of 2500 $\stackrel{o}{A}$ coincide with (n - 2)th fringe of 3500 $\stackrel{o}{A}$.
$\therefore 3500(n-2)=2500\times n1000n$ = 7000, n = 7
$\therefore$ 7th order fringe of 1st source will coincide with 5th order fringe of 2nd source.