1.

A point source of light is placed at a depthof h below the surface of water of refractive indexmu. Afloatingopaque disc is placed on the surface of water so thatlight from the source is not visible from the surface . The minimum diameter of the disc is

Answer»

`(2H)/((mu^(2)-1)^(1/2))`
`2h(mu^(2) -1 )^1/2`
`H/[2(mu^(2)-1)^(1/2)]`
`h(mu^(2)- 1)^(1/2)`

Solution :The figure shows incidence fromwater at critical angle ` theta_(C)`for thelimitingcase.
Now, ` SIN theta_(c) = 1/mu ` So that
` tan theta_(c) = 1/((mu^(2) -1)^(1//2))`
Fromfigure,` tan theta_(c) = r/h`
Where r is the radiusof the disc.Therefore of the disc is
` 2 r = 2 h tan theta_(c) =(2h)/((mu^(2) -1)^(1//2)`
` (##MTG_WB_JEE_CHE_MTP_01_E01_023_S01.png" width="80%">


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