A screen is placed 90cm from an object. The image of the object on the screen is formed by a convex lens at two different locations separated by 20cm. Determine the focal length of the lens.

Explanation

For `n^(th)` order bright fringe, path difference is, `(x_(n)d)/(D)=nlamda`
Here n=4 and so,
`(x_(4)d)/(D)=4lamda`
`:.lamda=(x_(4)d)/(D)`
`=((1.2)(0.28xx10^(-1)))/((4)(1.4xx10^(2)))`
`:.lamda=6xx10^(-5)cm`
OR `lamda=6xx10^(-5)xx10^(8)Å=6xx10^(3)Å=6000Å`