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## Homework Statement

An optical fiber consists of a glass core (index of refraction n1) surrounded by a coating (index of refration n2<n1). Suppose a beam of light enters the fiber from air at an angle θ with the fiber axis as shown in attached. (a) Show that the greatest possible value of θ for which a ray can travel down the fiber is given by θ = arcsin(sqrt(n1^2 - n2^2)). (b) If the indices of refraction of the glass and coating are 1.58 and 1.53, respectively, what is this value of the incident angle θ?

## Homework Equations

na*sin(θa) = n1*sin(θ1)

Critical Angle: θc = arcsin(n1/n2)

## The Attempt at a Solution

See attached for my drawing. Any angle greater than θc will cause total internal reflection so,

θo > θc

where θo = 90 - θ1

Substituting for θc

θo > arcsin(n2/n1)

Substituting for θo

90 - θ1 > arcsin(n2/n1)

Substituting for θ1

90 - arcsin(sin(θa)/n1) > arcsin(n2/n1)

Taking sine of the whole equation

1 - sin(θa/n1) > n2/n1

Solving for θa

θa < arcsin(n1-n2)

According to my calculations, this would be the maximum angle θa for which a ray can travel down the fiber. Unfortunately it's not correct.

Where do they get the sqrt(n1^2-n2^2) term?

Thanks in advance, MrMoose