This problem is out of the book as stated! A thin film of oil (n =1.25) is locat

Question

This problem is out of the book as stated!

A thin film of oil (n =1.25) is located on a smooth, wet pavement. When viewed perpendicular to the pavement, the film reflects most strongly red light at 640nm and reflects no green light at 512nm. How thick is the oil film?

I have no idea what the red and green light means and how do you tell which one is destructive or constructive interference and what does viewing the pavement at a perpendicular angle have to do with anything? I know there's already a solution but i need more explanation i guess on everything...

Explanation / Answer

For non-reflective coatings, then, the minimum film thickness required is:
t = (?coating/4) = (?vacuum/4n)
where n is the index of refraction of the coating material.
The wavelength of red light is ?red = 640 nm = 640 * 10^-9 m and n = 1.25 or t = (?red/4n) or t = (640 * 10^-9/1.25) = 512 * 10^-9 m = 512 nm The above gives the thickness of the oil flim.

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