In Young's experiment the wavelenght of red light is `7.8xx10^(-5) cm ` and that of blue light is `5.2xx10^(-5)` cm. The value of n for which `(n+1)^(th)` blue bright band coincides with `n^(th)` red band is

Answer

2

Explanation

Distance of the `n^(th)` bright band from the centre can be written as follows:
`y = n (lambda D)/d`
For bright bands to coincide, distance from the centre must be same for both. Hence we can write the following equation :
`y = (n + 1) (lambda_b D)/d = n(lambda_r D)/d`
`implies (n + 1)/n = (lambda_r)/(n) = (lambda_r)/(lambda_b)`
`implies (n + 1)/n = (780)/520 = 3/2`
`implies 2n + 2 = 3n`
`implies n = 2`