# Homework Help: Max Angle and Destructive Interference

1. Apr 24, 2010

### nightseeress

1. The problem statement, all variables and given/known data
Determine the maximum angle θ for which the light rays incident on the end of the the optical fiber of radius 1 mm are subject to the total internal reflection along the walls of the fiber. Assume that the fiber has an index of refraction of 1.3 and that the outside medium is air.

2. Relevant equations
sinθc = n2/n1

3. The attempt at a solution
sinθc = 1/1.3
θc = arcsin(1/1.3) = 50.28 degrees

However, the correct answer is 55 degrees. What am I doing wrong?

1. The problem statement, all variables and given/known data
A possible means for making an airplane invisible to radar is to coat the plane with an antireflective polymer coating. If the radar waves have frequency of 10GHz and the index of refraction of the polymer is n=1.5, how thick would you make the coating?

2. Relevant equations
2nt=mλ
c=fλ

3. The attempt at a solution
λ=c/f=(3x10^8)/(10x10^10)=0.003
2(1.5)t=(1)(0.003)= 1 mm

Correct answer is 5 mm. What am I doing wrong?