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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
`theta_("min") gt C`
`sin theta_("min") gt sin C`
`(R-d)/R gt 1/n`
`rArr Rn-dn gt R`
`rArr R gt (nd)/(n-1)`
`R gt (2. 4mm)/(2-1)`
`R gt 8 mm`
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RESONANCE-PART TEST 4-Exercise
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