Question

Prob. 6b (6 pts.) A glass window is coated with a transparent film of refractive index n2=1.25. This film will cause destructive interference in the reflected light when light of wavelength λ = 550nm. is incident normally,on the window. What is the minimum thickness of film required to produce this anti-reflection coating? incident 1 air fim glass 1.25

Answer 1

n_glass > n_film > n_air

     A phase change of 180^o will occur at both the air-film surface and the film-glass surface, since in both cases the

light is traveling from a material with lower index of refraction to a material with higher index of refraction.

Thus, the path length difference (2t) must be a half-integral number of wavelengths to get destructive interference

using the formula,

$2nt=(2n+1)\lambda /2$

n = 0, for minimum thickness

t = 550*10^-9/(4* 1.25)

t = 0.11*10^-6 m

the minimum thickness of film required is 0.11*10^-6 m