An optical fibre has diameter d and is made of material of refractive indeed `mu`. It is surrounded by air. Light is made to enter through one end of the fibre as shown. The fiber is in the shape of a circular bend of outer radius r.
(a) Find least value of `r (= r_(o))` for which no light can escape out of the fibre. Calculate `r_(o)` for `d = 200 mum` and `mu = 1.4`.
(b) How is value of `r_(o)` affected as d is made smaller?
(c) For sharper bends, shall we have higher `mu` or smaller `mu` ?

Answer

(a) `r_(0) = (mud)/(mu-1) 0.7 mm` (b) `r_(0)` gets smaller (c) higher `mu`.