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A cylinderical optical fibre (quarter ci...

A cylinderical optical fibre (quarter circular shape) of refractive index `n = 2` and diameter `d = 4mm` is surrounded by air. A light beam is sent into the fibre along its axis as shown in figure. Then the smallest outer radius `R` (as shown in figure) for which no light escapes after incident on curved surface of fibre is:
.

A

`2mm`

B

`4mm`

C

`8mm`

D

`6mm`

Text Solution

Verified by Experts

The correct Answer is:
C

`rArr R n - dn gt R rArr R gt (nd)/(n-1)`
`R gt (2.4 mm)/(2-1) rArr R gt 8mm`.
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Knowledge Check

  • A transparent solid cylinder rod has a refractive index of 2/sqrt(3) . It is surrounded by air. A light ray is incident at the mid point of one end of the rod as shown in the figure. The incident angle theta for which the light ray grazes along the wall of the rod is

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