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 2500 Ã… coincide with `(n-2)^(th)` fringe of 3500 Ã….
` :. 3500(n-2)=2500xx n`
`1000n=7000, " or " n=7`
` :. 7^(th)` order fringe of `1^(st)` source will coincide with `5^(th)` order fringe of `2^(nd)` source.