How Does Optical Fiber Cladding Affect Light Reflection Angles?

[emi_nic]

this is all making my head hurt![b(] any help for anything on here will be greatly appriciated!

-An optical fibre is made of a thin strand of glass with a refractive index of 1.5. If the fibre is surrounded by air, what is the maximum angle a ray of light may make with the glass-air boundary and still remain trapped in the fibre?
-The glass cores of optical fibres are norally surrounded by cladding of refractive index between 1.0 and 1.5. Explain whether this makes the maximum angle that a ray of light makes with the glass-cladding boundary bigger or small than the answer to the first question?







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...R1R2

(ignore the dots, they just idicate where spaces should be. Between the two vertical lines is the film, before those is the glass. the diagonal lines are the rays)

In order to cut out unwanted reflections and increase the percentage of light which is transmitted from a spectacle lens, a process called blooming is used. This involves a coating the lens with a very thin film of calcium fluoride of refractive index 1.45. The diagram shows tw0 reflected light rays, R1 and R2, of wavelength 580nm which will combine together to the right of the film.
-What is the minimum thickness of film which will give rise to R1 and R2 being in antiphase?
-Suggest why for this thickness of film, other wavelengths will not be completely cut out.

cheers.

 

[Valerie Park]

The minimum thickness of film which will give rise to R1 and R2 being in antiphase is given by the formula (n1-n2)*λ/2, where n1 and n2 are the refractive indices of the mediums on either side of the film and λ is the wavelength of the light. In this case, the refractive indices of the glass and air are 1.0 and 1.45, respectively, and the wavelength is 580 nm. This gives a value of (1.0-1.45) * 580nm/2 = -164.5 nm. This thickness of film will only cut out light rays at the exact wavelength of 580 nm, as any other wavelength would require a different thickness of film. This is because the thickness of the film required to shift the ray 180 degrees out of phase varies depending on the wavelength of the light.