Concept explainers
An optical fiber has an index of refraction n and diameter d. It is surrounded by vacuum. Light is sent into the fiber along its axis as shown in Figure P34.31. (a) Find the smallest outside radius Rmin permitted for a bend in the fiber if no light is to escape. (b) What If? What result does part (a) predict as d approaches zero? Is this behavior reasonable? Explain. (c) As n increases? (d) As n approaches 1? (c) Evaluate Rmin assuming the fiber diameter is 100 μm and its index of refraction is 1.40.
Figure P34.31
(a)

Answer to Problem 35.49P
Explanation of Solution
Given info: The index of refraction is n, the diameter of the optical fiber is d, it is surrounded by vacuum.
The figure that represents the given conditions is shown below,
Figure (1)
From figure a ray originally moves along the inner edge will have the smallest angle of incidence when it strikes the outer edge of the fiber in the curve. Thus, if this ray is totally internally reflected, all of the others are also totally reflected.
The necessary condition for the ray to be total internal reflection is,
θ0≥(θ0)c
Here,
θ0 is the angle of incidence.
(θ0)c is the critical angle.
For very small value of θ0.
The above condition can be written as,
sinθ0≥sin(θ0)c (1)
The expression of sin(θ0)c is,
sin(θ0)c=nairnpipe
Substitute 1 for the value of nair, and n for the value of npipe in the above expression.
sin(θ0)c=1n
The expression of sinθ0 is,
sinθ0=R−dR
Here,
R−d is the distance of optical fiber from the center.
R is the outside radius.
The expression of equation (1) becomes.
sinθ0≥sin(θ0)cR−dR≥1nR≥ndn−1.
The minimum value of outside radius permitted for a bend in the fiber is ndn−1.
Conclusion:
Therefore, the smallest outside radius Rmin. permitted for a bend in the fiber if no light is to escape is ndn−1
(b)

Answer to Problem 35.49P
Explanation of Solution
The result from part (a) is,
R≥ndn−1 (1)
Here,
R is the outside radius.
n is the index of refraction.
d is the diameter of the optical fiber.
The value of d becomes very small or goes to zero than Rmin will tends to zero from equation (1).
The lesser value of d indicates the optical fiber becomes thinner, if the optical fiber becomes thinner than the angle of incidence becomes very large and it strikes the surface at very large angle of incidence.
The thinner the optical fiber, the radius up to which the fiber is bent becomes smaller.
Conclusion:
Therefore, the Rmin→0 and yes, for very small d light strikes the interface at very large angles of incidence.
(c)

Answer to Problem 35.49P
Explanation of Solution
As n increases Rmin decreases from the result of part (a),
R≥ndn−1.
Here,
R is the outside radius.
n is the index of refraction.
d is the diameter of the optical fiber.
The above expression can be written as,
Rmin=ndn−1=d1−1n
It is clear that as n increases Rmin will decrease.
Yes, as n increases the critical angle becomes smaller
Conclusion:
Therefore, the Rmin decreases and yes, as n increases the critical angle becomes smaller.
(d)

Answer to Problem 35.49P
Explanation of Solution
As n increases Rmin decreases from the result of part (a),
R≥ndn−1.
Here,
R is the outside radius.
n is the index of refraction.
d is the diameter of the optical fiber.
The above expression can be written as,
Rmin=ndn−1
It is clear that as n approaches to zero, Rmin will tend to infinity.
Yes, as n approaches to 1, the critical angle becomes close to 90° and any bend will allow the light to escape.
Conclusion:
Therefore, the value Rmin tends to infinity and yes, as n approaches to 1, the critical angle becomes close to 90° and any bend will allow the light to escape.
(e)

Answer to Problem 35.49P
Explanation of Solution
Given info: The diameter of the fiber is 100 μm, and the index of refraction is 1.40.
Explanation:
The formula to calculate the Rmin derived from part (a) is,
Rmin=ndn−1
Here,
Rmin is minimum outside radius.
n is the index of refraction.
d is the diameter of the optical fiber.
Substitute 100 μm for d and 1.40 for n in the above formula.
Rmin=ndn−1=1.40×100 μm1.40−1=1.40×100 μm0.40=350 μm
Conclusion:
Therefore, the value of Rmin is 350 μm.
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