Question

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

Answer 1

Answer:

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