1.) When monochromatic light shines perpendicularly on a soap film ( n = 1.33) w
ID: 1382119 • Letter: 1
Question
1.) When monochromatic light shines perpendicularly on a soap film (n = 1.33) with air on each side, the second smallest nonzero film thickness for which destructive interference of reflected light is observed is 333 nm. What is the vacuum wavelength of the light in nm?
__________nm
2.) Orange light (?vacuum = 611 nm) shines on a soap film (n = 1.35) that has air on either side of it. The light strikes the film perpendicularly. What is the minimum thickness of the film for which constructive interference causes it to look bright in reflected light?
_________nm
Explanation / Answer
1) destructive interference is (2 n t) = m lambda
m = 2
wavelength of light in vacuum lambda = (2 n t) / 2
lambda =1.33 * 333
lambda = 442.9 nm
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2) n ---- refractive index , t --- thickness of the film
theta --- angle of incidence in the lower surface of the fillm , ?--- wavelength
2 n t cos theta = lambda / 2
t cos theta = constant
The maximum value of cos theta = 1.
2 n t = lambda / 2
tminimum = lambda/ 4 n
t = 611 * 10-9 / (4 * 1.33)
= 1.15 * 10-7 m = 115 nm
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