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A thin biprism (see Fig..) of obtuse ang...

A thin biprism (see Fig..) of obtuse angle `alpha=178^(@)` is places at a distance `l=20cm` from a slit. How many images are formed and what is the separation between them? Refrative index of the material `mu=1.6` .

Text Solution

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Two images are formed by the two thin prisms - one above the axis and the other below the axis by the same distance.
The refracting able of each thin prism is
`(pi-alpha)/(2)=(1)/(2)(pi-alpha)`
Then, `delta` (deviation of a ray) =`(mu-1)(1)/(2)(pi-alpha)`
`d/2=ldelta`
`d=2l(mu-1)(1)/(2)(pi-alpha)`
`d=(mu-1)l(pi-alpha)`
`d=(1.6-1)xx0.20(pi-178xx(pi)/(180))`
`= 0.6xx0.20xxpixx(1)/(90)=0.004m=4mm`
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