In double slit experiment fringes are obtained using light of wavelength `4800 Å` One slit is covered with a thin glass film of refractive index. `1.4` and another slit is covered by a film of same thickness but refractive index `1.7`. By doing so, the central fringe is shifted to fifth bright fringe in the original pattern. The thickness of glass film is

Options

A. `2xx10^(-3)` mm
B. `4xx10^(-3)` mm
C. `6xx10^(-3)` mm
D. `8xx10^(-3)` mm

Answer

D

Explanation

As per question,
`(mu-1)t(D)/(d)-(mu_(2)-1)(td)/(d)=(5lambdaD)/(d)`
`[(1.7-1)-(1.4-1)](td)/(d)=(5lambdaD)/(d)`
`:.t=(5lambda)/(0.3)=(5xx4800xx10^(-10))/(0.3)=8xx10^(-6) m =8xx10^(-3) mm`