# I Energy Loss During Total Internal Reflection

1. Mar 26, 2017

When a laser beam reflects during total internal reflection, how much of its intensity is lost?

I can't the use Fresnel equations as this is for total internal reflection.

If you don't know the answer to the above question, what about the same question, but for mirrors instead?

What are the parameters, and more importantly, how does the intensity lost change with the angle of reflection?

2. Mar 26, 2017

### Staff: Mentor

None. That is what the "total" means

3. Mar 26, 2017

but in the real world when you conduct experiments with let's say PMMA, the beam does get weaker with every reflection.

I also mentioned mirrors. Internal reflection within a block could lose energy just a like beam reflecting off a mirror loses energy.

And also this:

Last edited: Mar 26, 2017
4. Mar 26, 2017

### DrStupid

The beam gets weaker by absorption and scattering which hapens not only during the reflections..

5. Mar 26, 2017

true, but from my experience, the reflections do play a major role.

for example, when reflecting in a bar of similar dimensions as shown in the picture above, the increase in path length is not significant enough to produce the attenuation observed, and the beam gets noticeably weaker after each reflection.

6. Mar 26, 2017

### Staff: Mentor

Then to my understanding it isn't total internal reflection.

7. Mar 26, 2017

do you know of any formula that would describe the energy loss for reflection within PMMA or glass etc?

8. Mar 26, 2017

### Staff: Mentor

I believe what you are looking for is the Fresnel equations

9. Mar 26, 2017

The Fresnel equations that predict total internal reflection will likely never hold perfectly in practice. A good example of where the simple Fresnel formula gets an incomplete/incorrect answer is when you create a case of frustrated total internal reflection by putting a block of the material (usually done with triangular shaped pieces) parallel to and in close proximity of the surface with total internal reflection. As you bring the other block closer and closer, more and more of the beam gets sent through the second block, even though there is an air gap between the two blocks and Fresnel's formulas by themselves would predict total internal reflection. Using the complete Maxwell's equations on this new scenario explains the result completely. In any case, the Fresnel equations that describe total internal reflection, e.g. in a fiber optic cable, are not complete enough to explain reflection losses that will occur.

10. Mar 26, 2017

that sounds quite bizarre. do you have any links that describe this phenomenon in more detail?

11. Mar 27, 2017

It is most readily observed experimentally using microwaves where the blocks can be quite large and easily aligned. With optical wavelengths the experiment is more difficult to do, but the same result is found to occur. The Optics book by Klein discusses it. I think if you google "frustrated total internal reflection" you will get a discussion of it. $\\$ Editing... A google of it came up with several postings, and one described it as similar to a quantum mechanical type tunneling. When it is so readily performed with microwaves, I don't know whether it is quite at the level of "quantum mechanical tunneling", but in any case if you google it, you should find some interesting discussion of it. A complete explanation is provided using a classical wave description.

Last edited: Mar 27, 2017
12. Mar 27, 2017

Unfortunately, I can't apply Frustrated TIR as the scenario I have in mind has only one block of dielectric, not two.

Also, interesting that your posts are consistently 100-150 words long each. how come?

Last edited: Mar 27, 2017
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