Gemmology Question on Refraction.

[Pandora II]

Okay, I''m wading through the notes for my next assignment on Refraction.

They keep talking about ''optical density''.

I thought that optical density is a measure of a stone''s transparency and not a measure of its refractive index or of its mass.

I''m I going nuts here or am I just understanding badly.

Here''s a quote from the notes:

''The more the light is slowed when it enters a material, the more the material can ''bend'' the light. In other words, a greater amount of refraction will take place. Different materials tend to give different results for this refraction effect.

We can think of this effect as ''optical density'': the greater the slowing effect, the greater the optical density of the material. The greater the optical density, the greater the refractive power.''

I emailed my tutor,last night but haven''t heard back yet. I also spoke to my father (who used to be an opthamologist) and he thought I was probably right but he wasn''t 100% sure.

Any thoughts - at the moment I''m avoiding the phrase and getting round it by talking about ''light travelling through media of greater/lesser etc refractive index''.

 

[DiamondExpert]

Refractive index is, in fact, a measure of optical density. So a gem (or any substance for that matter) with a low optical density, would also have a low refractive index, and conversely. Optical density detemines how much light is slowed when passing through a substance, and, as I'm sure you are aware, is light frequency specific and the basis for dispersion.

Optical density should not, however, be confused with plain old density = mass/volume. There is not generally a relationship between the density of a substance and it's RI.

 

[Pandora II]

I understand what you are saying Gary, but the whole problem seems to be the actual meaning of the term ''optical density'' and whether it is correct in this usage.

For want of better words to put this, I''ve ''borrowed'' a few bits off the internet:

''I have taught university level courses on optics on numerous occasions and have never (as far as I remember) used the term
"optical density". I believe it is sometimes used to refer to the ability of a given substance to absorb light. Materials with large optical densities absorb light going through that material in a shorter distance than materials with smaller optical densities.

I believe the term is also sometimes used to describe the refractive index, but I think is a bad thing to do as it can easily lead to confusion. Refractive index is well-defined and says everything necessary about the refraction of light.

Best, Dick Plano, Professor of Physics emeritus, Rutgers University''

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(In answer to the question: What is the difference between optical density and refraction? I understand that optical density is related to the refractive index of a substance, but are they actually the
same thing, or is there a difference?)

Very different, from an initial-perception point of view. Optical index is how slow light goes through the medium (and hence how bent a ray will be);

Optical density is how darkening the medium is to light passing through.
I often wish I could use the phrase "optical density" meaning "index of refraction".
It seems a logical construction of English scientific language, and it fits in some sentences a little better.
But I guess it is taken.
When you see "optical density", think: How dense is the "dark smoke" in that smoky quartz?

Optical density number-scale takes a little getting used-to. It is typically numbered as a power of 10 of the light extinction ratio.

So a pair of medium-dark sunglasses that pass 10% of the light thru their lenses would have an optical density of 1, and probably an index of 1.5 to 1.6.

If you wore an identical second pair over the first, you would be using an optical density of 2, (but the index of the glass would still be 1.5).

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Not sure if this explains my dilemma better - I consulted Webster''s and he talks about ''optical density'' in reference to slowing of light speed as well. I''m probably just being majorly pedantic - as is my wont

But it''s really bugging me!

 

[DiamondExpert]

I think the Prof. hits it in his last sentence where he says that optical density is path length dependent, but RI is not.

So, while OD and RI are related for any given substance, they are not identical, and therefore the terms should not be, or at least are better not, used interchangeably to avoid confusion.