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 Post subject: Past Foundation exam questions, more info pls?
PostPosted: Sat Jan 09, 2010 3:05 pm 
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OK, in the run-up to the Foundation exam in a little over a week, I've been going through the past exam papers and filling in the answers.

One that I've run into more than once is:

"Why is YELLOW monochromatic light used with the refractometer?"

Now, I know why monochromatic light is used, and I know that the yellow is the equivalent of sodium light... but I'll be darned if I can find anything in my notes that explains why they actually use yellow monochromatic light as opposed to say... red or another colour.

If I were to guess, I would say that it's possibly because it was a wavelength that could be systematically reproduced using nothing but a flame and some salt.

Can anyone give me a solid answer on why they chose yellow?


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PostPosted: Sat Jan 09, 2010 3:19 pm 
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I'm thinking the answer may be as simple as readings with a yellow light source are easier to read than other colors of the spectrum. But that's just speculation.


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PostPosted: Sat Jan 09, 2010 3:38 pm 
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When were refractometers invented? Maybe it has to do with what they had available at the time?

I agree that yellow is likely easier to read.


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PostPosted: Sat Jan 09, 2010 4:05 pm 
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1874

The sodium yellow should keep the light's wavelength at about 580nm as a consistent wavelength as refractive index decreases when temperature/wavelength of the illumination increases.


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PostPosted: Sat Jan 09, 2010 4:24 pm 
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Ok, but if the baseline measurements weren't taken with a sodium light in the first place, any consistent wavelength would have been just as appropriate, no? So long as you're comparing apples to apples, right? Once they decided to use sodium light, it was a precedent and so it stuck. But the question is why they chose that particular light to start with...

There must be something special about sodium light that made it easier/better/more appropriate than the other choices that they must or may have had right at the beginning.


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 Post subject: Re: Past Foundation exam questions, more info pls?
PostPosted: Sat Jan 09, 2010 4:37 pm 
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africanuck wrote:
If I were to guess, I would say that it's possibly because it was a wavelength that could be systematically reproduced using nothing but a flame and some salt.


I think your initial idea here is more than likely dead on.


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PostPosted: Sat Jan 09, 2010 5:47 pm 
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Hi

Afri , i found this : If monochromatic light be used (i.e. the D line of the sodium flame) the field is sharply divided into a light and a dark portion, and the position of the line of demarcation on the scale immediately gives the refractive index.

maybe this is why this light sourse was used :?


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PostPosted: Sat Jan 09, 2010 6:10 pm 
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Thanks Drags,

I think that works for any monochromatic light though, not just sodium light. The RI readings might not be the same from one to another, but any monochromatic light should give a sharp line (as opposed to colour fringes with full spectrum light).


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PostPosted: Sat Jan 09, 2010 6:28 pm 
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Ok :oops: , maybe this

Make sure there is adequate amount of light. White light can be used while testing, but monochromatic yellow light is widely used. White light can give good results for single refractive stones, but for doubly refractive gems sodium light source is the best option. White light, when used with doubly refractive gemstones causes overlap of the refraction readings thereby getting wrong results. Also, the use of sodium light source clearly differentiates the boundary between dark and light areas and hence helps take the readings easily.


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PostPosted: Sat Jan 09, 2010 6:54 pm 
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But WHY, WHY, WHY??? :lol:

The thing is, they didn't add the YELLOW in there for no reason. From the assignments that we have done and from the advice given by my tutor, you really have to read the Gem A questions carefully and answer exactly what they ask for.

That YELLOW is in there and is important to the answer... otherwise they would have just said "monochromatic". They're sneaky like that... :wink:


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PostPosted: Sat Jan 09, 2010 7:07 pm 
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Sodium emits almost pure light at 589.3 nm (sodium ā€˜Dā€™ line) or two spectral lines at 589.1 and 589.6 nm so close together they're regarded as a pure light source with average of 589.3 nm. The consistency of the wavelength makes it ideal and refractometers are configured to it. I imagine back then it was no easy feat to achieve monochromatic light using candles etc so so the sodium method was adapted because of the simplicity, clear readings and repeatability.

From "Gemmology" by Peter G. Read
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For accurate work it is best to use a monochromatic sodium light source, as this gives the sharpest and most easily seen shadow edge


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PostPosted: Sat Jan 09, 2010 9:45 pm 
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africanuck wrote:
I think that works for any monochromatic light though, not just sodium light.


I can't guarantee this is the answer they are looking for, but it seems like the only sensible answer to me: before the 1960s, the yellow light produced by a sodium lamp was the only monochromatic light available. And a sodlum lamp is still the only source of unfiltered monochromatic, incoherent light available. To put it another way, there is no other lamp that produces incoherent light at only a single wavelength (or over only a short [say, less than 1nm] range of wavelengths).

Before lasers came around in the 1960s, a sodium spectral lamp was the only monochromatic light source available. I say monochromatic as if you can count the two D lines as one, and for most situations you can, so that sodium lamp emission is around 99% pure D line. And these lamps are very intense, very bright. The fact that you can produce the yellow light with salt in a candle flame is just a bonus.

Even today, one might think of changing the standard to a common laser color, such as green 532nm light from a DPSS laser. But laser light, along with being monochromatic, is also coherent. Coherence can cause some funny interference problems like speckle that aren't conducive to producing sharp lines. These interference effects don't occur with the nearly monochromatic, but incoherent, sodium light.

Nowadays, with interference filters, you can filter any light source down to as narrow a band as you want. But a filter subtracts light, it is not a source of light.


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PostPosted: Sun Jan 10, 2010 5:40 am 
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Thanks Brian, that certainly sounds like a WHY to me :D

Thanks to everyone else who helped with this as well, much appreciated.


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PostPosted: Wed Mar 17, 2010 8:15 pm 
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Bit late here, but I think the answer may be this:

All the tables of RI values are based on using yellow monochromatic light. If you don't use yellow then your readings will not correspond correctly with the tables. Therefore all refractometers are used with yellow light.

The 'why' for the original use of yellow monochromatic light is probably not what they are after.


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 Post subject: Re:
PostPosted: Mon Aug 09, 2010 7:59 am 
Brian wrote:
africanuck wrote:
I think that works for any monochromatic light though, not just sodium light.


I can't guarantee this is the answer they are looking for, but it seems like the only sensible answer to me: before the 1960s, the yellow light produced by a sodium lamp was the only monochromatic light available. And a sodlum lamp is still the only source of unfiltered monochromatic, incoherent light available. To put it another way, there is no other lamp that produces incoherent light at only a single wavelength (or over only a short [say, less than 1nm] range of wavelengths).


Take a look at this: http://www.youtube.com/watch?v=jJvS4uc4 ... ature=fvwp.

That the 'burning' of different salts creates different coloured lights is information probably as old as the Chinese invention of fireworks. The use of a 'flame test' in the identification of the presence of a certain elements, each being associated with a particular colour of flame, goes back at least to the 18th century.

Some elements e.g. Calcium emits light at so many wavelengths, the light appears to be white, others emit strongly in one colour band and but weakly at others (see http://en.wikipedia.org/wiki/Emission_spectrum) ). The idea that any incandescent source can produces radiation only at one exact wavelength is a convenient concept but not exact for more than one reason (multiple emissions at different wavelengths but in the same 'colour' band spread, presence of impurities, etc). Even modern Sodium lamps emit at more than one wavelength but these wavelengths are close together in the 'yellow' band - but Sodium light was chosen well before this could have been known. See http://en.wikipedia.org/wiki/Sodium-vapor_lamp . So why was Sodium light chosen?

The fathers of refractometry (who had only pre-electric incandescent light sources) realised that a monochromatic (not the same thing as a single wavelength light source - which would have been unavailable to them anyway) was very preferable for use in refractometry. They could have used (say) a Strontium salt rather than Sodium salt but chose the latter. Why? I'd guess because of:
- Ubiquitous availability and cheapness of Sodium Chloride.
- The human eye is more sensitive to yellow light than it is to any other (i.e. yellow light looks brighter)

Whatever, it's self-evident that all refractometrists need to use same colour of light or else results are not repeatable from one party to the next. A green light source could, theoretically, be the best as that is the colour band central to the spectrum of visible light but Sodium emissions both appear brighter for any given power and are more central to the visible spectrum that (say) those of Strontium - and are cheap to produce.


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