1.

In a Young's double slit experiment, d=0.5 mm andD = 100 cm. It is found that 9^(th) bright fringe is at a distance of 7.5 mm from the second dark fringe of fringe pattern. The wavelength of light used is (in Å)

Answer»

`2500/7`
2500
5000
`5000/7`

Solution :For BRIGHT FRINGES , `x=(nlambdaD)/d`
where n=0,1,2,3,…
For dark fringes , `x=((2n-1)lambdaD)/(2d)`
where n=1,2,3,…
As PER question
`(9lambdaD)/d-(3lambdaD)/(2d)=7.5xx10^(-3)m` or `(15lambdaD)/(2d)=7.5xx10^(-3)` m
`LAMBDA=(2xx7.5xx10^(-3) m xxd)/(15D)`
Substituting the given values , we GET
`lambda=(2xx7.5xx10^(-3) m xx 0.5xx10^(-3) m )/(15xx1 m)` = 5000 Å


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