refraction of light

Question

True or false? light traveling from crown glass to asphalt never experience total internal reflection.

True or false? Light traveling from whale oil to vacuum can never experience total internal reflection.

What is the critical angle for light traveling from salt to water? If no critical angle exists, type 'none'. Round your answer to a tenth of a degree.

Answer 1

Hi Noodles D.,

As you probably already know(?), you start with Snell's Law : n₁sinθ₁ = n₂sinθ₂ . To have total internal relection, you need sinθ₂ > 1 (so that the exit ray would have to be refracted to *lower* than the surface). That will occur with the right combination of sinθ₁ and n₁/n₂ . In particular, it can only happen on passage from a higher to a lower index of refraction. Look up your indices of refraction, and start calculating (anything to a vacuum can have T.I.R., since vacuum has n=1).

Strangely enough, the light which will be totally internally reflected, "samples" the medium it does not totally emerge into. If there is something there which absorbs certain frequencies of light, some of those frequencies can be at least partially absorbed, and therefore be weaker or missing in the totally internally reflected light beam.

This can also happen with ordinary reflected light, i.e. reflected light from a solid. That's the reason that the metal gold is yellow colored: the reflected light "samples" into the metal, and the blue portions are absorbed. (in thin layers of gold, which are thicker than the zone sampled by reflected light, more of the spectrum is absorbed, and the transmitted light color is red).

-- Cheers, --Mr. d.