Energy Loss During Total Internal Reflection

[tade]

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?

 

[Dale]

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

None. That is what the "total" means

 

[tade]

None. That is what the "total" means

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:

...more importantly, how does the intensity lost change with the angle of reflection?

 

[DrStupid]

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

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

 

[tade]

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

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.

 

[Dale]

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.

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

 

[tade]

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

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

 

[Dale]

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

I believe what you are looking for is the Fresnel equations

 

[Charles Link]

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.

 

[tade]

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.

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

 

[Charles Link]

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

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.

 

[tade]

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.

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?

 

[tade]

I believe what you are looking for is the Fresnel equations

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 silver mirrors instead?

 

[tade]

I think the answer lies in the energy loss die to evanescent waves.

I managed to find this page: http://www.olympusmicro.com/primer/techniques/fluorescence/tirf/tirfintro.html

The intensities of the evanescent waves. If only there was a way to convert it into transmittance.

 

[tade]

bump in case someone who knows is around