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Lesson: Thin Film Interference

Worksheet: Thin Film interference • 9 Questions

Q1:

A thin film with a refractive index of 1.32 is surrounded by air. What is the minimum thickness of this film that minimizes the reflection of normally incident light of wavelength 500 nm?

Q2:

After a minor oil spill, a thin film of oil, with a thickness of 450 nm, floats on the surface of the water in a bay. Use a value of 1.40 for the refractive index of the oil, and a value of 1.33 for the refractive index of the water. What is the predominant wavelength seen by someone flying in a helicopter overhead?

Q3:

A transparent film of thickness 250 nm and index of refraction of 1.40 is surrounded by air. A beam of white light that is incident almost normal to the film undergoes destructive interference when reflected. What wavelength in the beam undergoes destructive interference at reflection from the film?

Q4:

A film of oil that has a refractive index of 1.40, floating on water will appear dark when it is very thin because the path length difference between the upper and lower boundaries of the film becomes small compared with the wavelength of light and there is a phase shift at the top surface. If the film of oil on some water becomes dark when the path length difference between the film's upper and lower boundaries is less than one-fourth the wavelength, what is the thickest the oil can be and appear dark at all visible wavelengths?

Q5:

A good quality camera “lens” is actually a system of lenses, rather than a single lens, but a side effect is that a reflection from the surface of one lens can bounce around many times within the system, creating artifacts in the photograph. To counteract this problem, one of the lenses in such a system is coated with a thin layer of material with a refractive index on one side. The index of refraction of the lens glass is 1.68. What is the smallest thickness of the coating that reduces the reflection at 640 nm by destructive interference? (In other words, the coating’s effect is to be optimized for .)

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