Optics Question 844
Question: A possible means for making an airplane invisible to radar is to coat the plane with an anti-reflective polymer. Radar waves have a wavelength of 3.00 cm and the index of refraction of the polymer is n=1.50. How thick would you make the coating?
Options:
A) 1.50cm
B) 3.00cm
C) 0.50cm
D) None of these
Show Answer
Answer:
Correct Answer: C
Solution:
[c] Treating the anti-reflectance coating like a camera-lens coating, one can obtain for its thickness t: $ 2t=( m+\frac{1}{2} )\frac{\lambda }{n} $ , $ m=0,\pm 1,\pm 2 $ …… (i)
Condition (1) is the condition for the destructive interference for the normally incident and reflected electromagnetic wave.
The minimum thickness of the coat refers to m= 0.
This gives for the thickness: $ t=\frac{\lambda }{4n}=\frac{3.00}{4(1.5)}cm=0.500cm $
This anti-reflectance coating could be easily countered by changing the wavelength of the radar to L50 cm now creating maximum reflection.
