Homework Help: Optics - Refracting light on film oil

1. May 16, 2007

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 dont understand this phenomena.

I tried to explain it through dispersion, but dont think dispersion can happen at normal incidence.

Would Some one please explain the physics behind this.

2. May 16, 2007

Staff: Mentor

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" [Broken]

Last edited by a moderator: May 2, 2017
3. May 16, 2007

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 dont change with with interference or doesnt it. Please correct me if i am worng?

4. May 16, 2007

Staff: Mentor

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.

5. May 16, 2007

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?

6. May 16, 2007

Staff: Mentor

Last edited by a moderator: Apr 22, 2017