A film of transparent material 120 nm thick and having refractive index 1.25 is placed on a glass sheet having refractive index 1.50. (A) Determine the longest wavelength of light that interferes destructively when reflected from the film. (B) Determine the longest wavelength that interferes constructively

a) 600nm

b) 300nm

Explanation:

the path difference = 2t  

t = thickness of the film

L' = wavelength of light in film = L/n

L = wavength of light in air

n = refractive index of glass

(a)

for destructive interference 2t = L'/2 = L/2n

L = 4*t*n

= 4*120*10^-9*1.25  

L = 600 nm

(b)

for constructive interference 2t = L' = L/1.25

L = 2tn

= 2 × 1.25 ×  120nm

= 300 nm

the correct choice is a .

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