In a lab a film of transparent oil with index of refraction 1.29 floats on top o

Question

In a lab a film of transparent oil with index of refraction 1.29 floats on top of a calm beaker of water (index of retraction 1.33). Above the oil is, well just air, and below it is a beam of a Helium-Neon laser (wavelength 632.8 nm in vacuum - red) coming straight up out of the water. When the beam hits the oil and air it undergoes reflection back down into the water. Suppose this reflection is minimal due to destructive interference, what is the thinnest the oil can be and have this happen? cc a) 115 nm b) 120 nm c) 125 nm d) 130 nm e) 135 nm

Explanation / Answer

wavelength of light in water = 632.8/1.33

path difference = 2t

for destructive interference path difference = m*lambda/2

2t = m* m*lambda/2

t = m*lambda/4

t = m*632.8/(4*1.33)

t = 120 nm <<-------answer

option b

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