In young's double slit experiment 'the wavelength of red light is`7800Å` and that of blur light is `5200Å`.the value of n of which `nth` bright band due to red light coincides with (n+1) bright band due to blue light is:

Explanation

`(n_(R)lambda_(R)D)/(d) = (n_(B)lambda_(B)D)/(d)` or
`(n_(R))/(n_(B))= (lambda_(B))/(lambda_(R)) = (5200)/(7800) = (2)/(3)`
Therefore `2^(th)` of red coincides with `3^(th)` of blue .