Light Refraction: Which Material Takes Longest?

[baileya]

Homework Statement

If we have 3 materials with the same thickness but different indices of refraction (1st= 1.33, 3rd=2.419) which would take the longest time for light to pass through? Why?

Homework Equations

n/a

The Attempt at a Solution

I want to say it's the third one simply because it has the higher n value, but I'm unsure as to why? Is it because the higher n value causes the light to bounce around inside the material a lot before leaving?

 

[tedbradly]

The problem is impossible to answer. It does not state that the light is approaching at the same angle or through the same median before it comes in contact with the three materials. And then we can think of a situation where light was traveling at 90 degrees to the normal originally for cases (1) and (3) yet traveling at 0 degrees to the normal in case (2), in which case the finite time from (2) is faster than the infinite times for cases (1) and (3). Similarly, we could then think of an instance where light is traveling at 90 degrees to the normal for cases (1) and (2) but at 0 degrees for case (3). Then, (3) would be the answer using the finite-infinite argument.

Do you see the arbitrary nature of the question's wording?

Surely, however, it seems to presuppose the same medians and same original angles, but we could then say, since the question has not defined it, what if I chose the original angle to be 90 degrees? Then, the answer would be, "none are faster," because all times would approach infinity.

 

[gneill]

http://en.wikipedia.org/wiki/Refractive_index"

With the assumption that the incident angle for the light is zero degrees to the normal, then the traversal time is determined by the speed of light in the various media. The speed of light is intimately associated with the index of refraction, as you will see in the above link.

 

[atkinson75]

Is there a minimum thickness for refraction to occur? In other words, can you ignore a very *very* thin layer (20nm or so) of a material that is even a high index (like n=2) in between two layers of lower index? You would still get potential TIR at high-low index interface, right?

Thanks.

 

[gneill]

I think that for *very* thin layers you'd have to look more deeply into the physics of what happens at the surfaces of the materials. At those scales, how can you tell for sure when you've left one material and entered another?

 

[fisixC]

gneill will you please help me with my question?
I'm so desperate and lost..
https://www.physicsforums.com/showthread.php?t=475627

 

[atkinson75]

gneill, I'm not talking about atomic level thicknesses .. but let's say a 10th of the wavelength, or 20nm. Is this too thin to cause refraction? Is the ray path altered transmitting through this layer?

 

[gneill]

gneill, I'm not talking about atomic level thicknesses .. but let's say a 10th of the wavelength, or 20nm. Is this too thin to cause refraction? Is the ray path altered transmitting through this layer?

I would say that, yes, the ray path will be altered. I suppose whether or not the path change is significant would depend upon what equipment you have to measure it!

I hear that the refractive index of dielectric thin films depends upon thickness. Now, what constitutes the thickness of said films, I don't know. Presumably at least several atomic layers.