In young's double slit experiment, distance between the slits is `1mm. The fringe width is found to be 0.6 mm. When the screen is moved through a distance of 0.25 m, the fringe width becomes 0.75 mm. Find the wavelength of light used.

Explanation

Given `d=1xx10^(-3) m, beta=6xx10^(-4) m, D'=D+0.25`
We know that `beta prop D. ` Hence `(beta')/(beta)=(D')/(D)=(D+0.25)/(D)`
or `(7.5xx10^(-4))/(6xx10^(-4))=(D+0.25)/(D)`
i.e., `5D=4D+1`
`therefore D=1m`
Substituting the values of `beta, d and D`,
`lambda=(betad)/(D)=(6xx10^(-4)xx1xx10^(-3))/(1)`
i.e., `lambda=6xx10^(-7) m `
or `lambda=6000 overset(@)A`