Could be a daft question but needs asking

• resurgance2001
In summary, the student was pointing out that there is still some light escaping from the prism even though the incident ray of light is well beyond the critical angle. This is due to the fact that the prism has an evanescent field which drops in intensity exponentially as you move away from the surface.
resurgance2001
I was trying to demontrate to some GCSE student total internal reflection, when one of the students pointed out that what I was calling total internal reflection was not really total because there was actually still some light escaping from the plastic prism we were using.

Was this just due to the fact that the prism was a rather grotty perpex affair? I changed to a very nice brand new glass one, then a parallel sided block and then a semi-circular one, all the time the said smidgen of light still plainly visible even though the incident ray of light was well beyond the critical angle and there was no emergent refracted ray only the so-called totally internally reflected one.

Can anyone explain the above? Is it just the case the even with my nice new glass prisms they are not really optically flat or is there something else at play here?

Thanks

Peter

There's no "total" anything when it comes to reflection or refraction, etc. Some light will escape as the students observed. Some light will also be absorbed by the material too. Even in a plane mirror, some light is being absorbed and even refracted through the mirror.

How were you detecting the emergent ray?

It looks like he's referring to the faint spot that you can see on the face of the prism itself, where the beam is "totally" reflected inside the prism. I think this is due to microscopic irregularities in the surface of the prism.

When you get total internal reflection, the beam actually penetrates through the glass (or perspex or whatever)/air interface slightly, this is what's called an evanescent wave, or evanescent field. The evanescent field drops of in intensity exponentially as you move away from the surface. Typically an evanescent field will at most penetrate about 1 micron through the interface. **DO NOT CONFUSE THE EVANESCENT FIELD WITH A REFRACTED BEAM** Think of the totally internally reflected beam going through the boundary slightly before being reflected back.

What this means is that light will actually scatter off the surface, dust that happens to be on the surface, even the surrounding air molecules, because the beam is not wholly contained within the solid medium. This is where the observed 'smidgen' of light is coming from.

Interestingly, this is one reason why optic fibres have a cladding layer, rather than just a naked core.

Claude.

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These are interesting responces. The thing that was bothering me about this is that I'd been taught that we use prisms in situations where the most accurate reflection is required and the the efficiency of prisms is something like 99% as opposed to something like 95% for regular mirrors. Whatever I'm guessing at these numbers but it seemed to me that with that much light 'escaping', how could prisms be that effiicient for the purposes of reflection?

Claude Bile's answer about evanescent fields is very very interesting and I would like to pursue that line of enquiry further. So thank you very much.

Cheers

Peter

Here is a neat little discussion on evanescent fields you might find interesting.

http://www.physics.harvard.edu/~tomhunt/pubs/evanescent.pdf

Claude.

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1. What is a "daft" question?

A "daft" question is a question that may seem silly or foolish, but it is still worth asking because it can lead to important insights or clarifications.

2. Why is it important to ask questions, even if they may seem silly?

Asking questions is an important part of the scientific process. It allows us to gather information, challenge assumptions, and discover new knowledge. Even "daft" questions can help us think outside the box and uncover valuable insights.

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