Refractive index of optical fibre

[xts]

How could this happen?does the refractive index of the dirt higher than that of the core?

No. Read (e.g. wiki: http://en.wikipedia.org/wiki/Evanescent_wave) about TIR mechanism or very good explanation in Feynman Lectures on Physics.
The light is not reflected "sharp" on the boundary between media. The light enters to the lower index medium on some depth (order of magnitude its wavelength). If the lower index medium is not transparent, the light gets absorbed or scattered, rather than reflected.

It is a perfect demonstration: you send a laser beam through a glass prism such, that it is reflected by TIR. Then you blow some cigarette smokeinto the area behind the "reflecting" surface. You may see the smoke glowing, and if you have precise photometer, you may also notice that reflected beam gets a bit dimmer.
Similar demonstration is with total reflection between glass and water - then you add some milk or ink to the water: in both cases the reflected beam gets dimmer to such extend you may see it by naked eye.

you mean high??hmm that's confusing

Almost parallel to fibre axis. Even in multimode fibre the angle is much lower than 1°.

 

[xts]

And one more school demonstration: take a glass prism (45°/45°/90°) and look through shorter side - then you see reflection by TIR of the object behind other short side.
Then touch the long side (reflecting) with a finger - you now may see very clearly the fingerprint mixed with the reflection of what you were watching. That visible darkening of the reflected light in some places is caused by light loss on so small amount of dirt behind reflecting surface, as a grease left by fingerprint.

 

[sophiecentaur]

There is energy 'in' the low index medium. But it's in the form of an evanescent wave, which doesn't propagate. A lossy medium will absorb this energy, right in close to the interface.

 

[Misr]

Almost parallel to fibre axis. Even in multimode fibre the angle is much lower than 1°.

I think you mean that the angle of incidence is very big

 

[Misr]

A read another explanation for the function of the cladding:
The cladding is of higher refractive index than air and lower than that of the core
so if some light rays fell at an angle less than the critical angle,it will refract to the cladding and the cladding will reflect them back to the core
Is that true?

 

[Misr]

ok one more question,
can light penetrate the cladding?what happens in this case?

 

[xts]

@Misr,
it would be really great, if you read answers you already got to your questions before asking the same question again.
Be so nice and read answers #36 and #38, then follow a link to wiki you got.