Optics - Refracting light on film oil

[iontail]

Hi i was going through some of my physics past papers to prepare for exams, when i came across this question

A film of oil of refractive index 1.45 rests on a flat piece of glass of refractive index 1.6. When illuminated by white light at normal incidence, light of wavelengths 690 nm and 460 nm are predominant in the reflected light. I don't understand this phenomena.

I tried to explain it through dispersion, but don't think dispersion can happen at normal incidence.

Would Some one please explain the physics behind this.

 

[Doc Al]

The optical properties of thin films are due to the interference of light reflecting from the boundaries. Read this: http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/thinfilm.html"

 

[iontail]

I inderstand the cpncept of interference, but my problem is i cannot explain why i observe two waves of different wavelenghts, as interference is caused by phase difference, and i believe the wavelength of waves don't change with with interference or doesn't it. Please correct me if i am worng?

 

[Doc Al]

Treat each wavelength in the incoming light separately. For each wavelength, figure out the phase difference between the light reflecting from (1) top layer of the film (air/oil) and (2) bottom layer of the film (oil/glass). That phase difference depends on the wavelength of the light and the thickness and index of refraction of the film. Wavelengths that constructively interfere will be predominant in the reflection light; wavelengths that destructively interfere will not be.

 

[iontail]

so the fact that white light is made of electromagnetic radiation of different wavelength is involved in our understanding of the situation? I was say this to my friend, but he said it was not correct. Was i right when i thought of white light being made of electromagnetic radiation of different colours?

 

[Doc Al]

You are correct. The fact that white light is a mix of all wavelengths is key to understanding this. I assume you've seen a prism in operation? http://hyperphysics.phy-astr.gsu.edu/hbase/ems3.html#c2"