Two sources of light of wavelengths 2500 Å and 3500 Å are used in Young’s double slit expt. simultaneously. Which orders of fringes of two wavelength patterns coincide?

Options

A. `3^(rd)` order of `1^(st)` source and `5^(th)` of the `2^(nd)`
B. `7^(th)` order of `1^(st)` and `5^(th)` order of `2^(nd)`
C. `5^(th)` order of `1^(st)` and `3^(rd)` order of `2^(nd)`
D. `5^(th)` order of `1^(st)` and `7^(th)` order of `2^(nd)`

Answer

B

Explanation

Let `n^(th)` fringe of `2500Å` Coincide with `(n-2)^(th)` fringe of `3500Å`.
`:.3500(n-2)=2500xxn` 0r `1000n=7000`, or `n=7`
`:.7^(th)` order fringe of `1^(st)` source will coincide with `5^(th)` order fringe of `2^(nd)` source.