What is the refraction index of liver?

[LT72884]

Homework Statement

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Ok, so i finished my lab for physics 2 today. It was on reflection, refraction, Snells law etc. During my study, i looked up the refraction index of glass via this link: https://en.wikipedia.org/wiki/List_of_refractive_indices

i noticed it had refraction indexes for a number of things that do not make senses to me, such as liver, or other tissues. We used laser pointers and prisms for our lab, but is it same to assume that the refraction index of liver is not for visible light like a laser?

Thanks

 

[Merlin3189]

In that Wiki article, the index for liver is specified at 964 nm in the infrared region.
It may have more transparency at that frequency.

Edit: PS, I'd guess you could get a laser at that frequency. And probably a camera.

Edit 2: Yes you can.
Also, if you look at the Wiki page you quote, the paper, from which they got the refractive index for liver, is quoted and you can look at it on the internet. It explains a bit about finding refractive inices in not very transparent media.
It is interesting, though rather advanced for me. They are looking for changes in refractive inex in cancerous tissue to enable immediate determination in the course of an operation, rather than having to send a sample for histology, with maybe days delay.

 

[LT72884]

In that Wiki article, the index for liver is specified at 964 nm in the infrared region.
It may have more transparency at that frequency.

Edit: PS, I'd guess you could get a laser at that frequency. And probably a camera.

Edit 2: Yes you can.
Also, if you look at the Wiki page you quote, the paper, from which they got the refractive index for liver, is quoted and you can look at it on the internet. It explains a bit about finding refractive inices in not very transparent media.
It is interesting, though rather advanced for me. They are looking for changes in refractive inex in cancerous tissue to enable immediate determination in the course of an operation, rather than having to send a sample for histology, with maybe days delay.

Thats actually really cool though. Thanks for pointing that out. I sometimes forget about the references

 

[Merlin3189]

I do myself. And they're often behind pay walls.

BTW, although the index given in the Wiki table is for 964 nm IR, they did test it with other, visible wavelengths and they give the refractive index for those.

I'm glad I looked at your question, because it got me thinking.
I was wondering whether perhaps they were using a very thin sample, as you might when you make a microscope slide. So if liver was a little transparent, maybe enough light gets through a very thin slice.
Then I saw they were using critical angle and total internal reflection to calculate the index. So I thought, the light doesn't have to go through the liver at all! And that made me wonder, how does it know that the next medium has a lower refractive index, if it doesn't actually go into it?
So I ended up reading Feynman to find out how TIR works and nearly choked on the maths!
The upshot seems to be, you need only about a few wavelength thickness of the second medium. If the light is absorbed, so much the better. (Don't take my word for this. Read it yourself if you want to know. I may well have misunderstood.)

 

[DaveE]

There are several biophotonic applications that involve transmitting laser light through tissues. Yes, to us they don't look transparent, but if the light is bright enough, photons do get through, or reflect. BTW, 964nm is a really common wavelength for high power laser diodes.

For example:
"The magnitude of the real and imaginary part of the refractive index is correlated with hepatic pathology. Notably, the real index contrast is pointed out as a marker of discrimination between normal liver tissue and hepatic metastases. In view of the current progress in optical biosensor technologies, our findings may be exploited for the development of novel surgical and endoscopic tools...

Determining the refractive index of biological matter is needed for biophotonics applications including laser therapy, optical diagnosis and biomedical imaging. Recently, a limited number of studies have also pointed towards the direct use of refractive index as a marker for distinguishing between normal and malignant human breast and prostate1, as well as rat brain tissues2..."

From this paper:
https://www.nature.com/articles/srep27910

 

[hutchphd]

The upshot seems to be, you need only about a few wavelength thickness of the second medium. If the light is absorbed, so much the better. (Don't take my word for this. Read it yourself if you want to know. I may well have misunderstood.)

It appears the coupling is through the face of a prism onto the liver tissue. One needs to realize the relevant scales here: The size of an atom is roughly 0.1nm so the decay depth of this light comprises a layer 10,000 atoms thick on the liver. Is this small? Should be enough.