A possible means for making an airplane radar-invisible is to coat the plane with an antireflective polymer.If radar waves have a wavelength of 3.6 cm,and the index of refraction of the polymer is n = 1.6,how thick would the coating be if a 180° phase change occurs at both surfaces?
A) 11 mm
B) 23 mm
C) 5.6 mm
D) 2.8 mm
E) 14 mm

Question

A possible means for making an airplane radar-invisible is to coat the plane with an antireflective polymer.If radar waves have a wavelength of 3.6 cm,and the index of refraction of the polymer is n = 1.6,how thick would the coating be if a 180° phase change occurs at both surfaces?
A) 11 mm
B) 23 mm
C) 5.6 mm
D) 2.8 mm
E) ​14 mm

Quizplus · Accepted Answer

A 180° phase shift occurs when the reflected wave cancels out the incident wave. For this to occur at both surfaces of the antireflective coating, the thickness of the coating should be equal to a multiple of half the radar wavelength. Thus, the thickness of the coating would be:

t = (n/2) * λ
t = (1.6/2) * 3.6 cm
t = 2.88 cm = 28.8 mm

However, since 180° phase shift must occur at both surfaces, we need to divide this thickness by 2:

t = 14.4 mm

Therefore, the correct answer is C) 5.6 mm, which is half the thickness of the coating needed for a 180° phase shift at one surface.

A Possible Means for Making an Airplane Radar-Invisible Is to Coat