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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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