Measuring Refractive Index of Glass Using Gem RefractometerSummary of Activity: In this activity we will use the GemRefractometer to measure the refractive index of common glassymaterials. To use the method the sample must have one flat surface.In this lab we provide a collection of glassy materials of differentrefractive index for the experimenter to measure and compare withknown values. The Gem Refractometer can be purchased for approximately 100 and provides avery easy and convenient apparatus for measuring the refractive index of a wide range ofmaterials with at least one flat surface, which includes prisms, plano lenses, panes and flats.Overview of Method:The method is based on the direct application of Snell’s law and the critical angle, above whichtotal internal reflection occurs. For θ above θc we will observe essentially 100% back reflectionfrom the higher index medium side.Thus if a divergent light sourceimpinges on the interface throughthe higher density material from theleft as shown in the figure, thenthere will be an abrupt increase inthe brightness of light reflected backat the critical angle. Below thecritical angle most of the light willbe transmitted and not reflected; thusthe region below the critical anglewill appear darker in reflection.Recallsin θc n1/n2This effect is easiest to observe through a high index hemisphere where it is easy to observe theback reflected light. The Gem Refractometer utilizes such a high index, “hemispherical” lenswith an attached or built in light source to facilitate the measurement. A special high index oil isused to preclude a glass/air interface which would disrupt the phenomena. Additional details onthe actual gem refractometer are provides in an appendix.GemRefractometerMeasurements.docxHeffner 1/31/121
How to Use the Gem Refractometer:Photo ctions.htmNote: Always wash your hands after you make physical contact with the refractive index (RI)liquids. The MSDS list no health hazard at Condition of Intended Use but the oil should not beinhaled and avoid contact with skin for good measure. See MSDS and note 6 for details.Directions for Use:1. Make sure the lamp is attached (if not internally lighted), batteries fresh and switch on.2. Put a tiny drop of RI liquid on the metal plate near the hemicylinder. You only need to place areally small drop of the solution -- just enough to cover a small portion of the metal base near thehemicylinder.3. Place the glass sample right on top of the RI liquid and slide it onto the cylinder withoutbreaking the seal between the surfaces. Center the sample and RI on the hemicylinder.4. View the reading scale through the eyepiece at about 6” away so you can see the scale insidethe refractometer. With practice, you will find the right spot to use for your eyes. Note the pointon the scale where the dark to light transition line rests; this will be the measure for yourrefractive index.For more detailed procedures or to learn about a You Tube video on the method see a discussionin the end notes.GemRefractometerMeasurements.docxHeffner 1/31/122
Gem Refractometer Experimental Activity:Using the method on the previous page determine the refractive index of the samples providedby you instructor and listed in the table below. Make 2 measurements to check repeatability.Samplematerial typeExpectedMeasured IndexIndex1st2ndAvg.flint (lead) glass1.62F2 - Prismhigh dispersion(L1885D)BK7 prismcrown glass,1.517(L1851D)alkalai, Ca silicawindow glasssoda lime silica1.520LEBG sq. blanklow expansion1.47(PM1250)borosilicate glassFused silicasilica glass1.46window(PM1078)acrylic sheetpoly styrenesheetpmma1.491.59unknownsmicroscope slidehard candyFor source of data used in the above table see note 3. Indices listed are for 587 nm. See note 4Make a graph of measured value vs. expected and comment on how well they compare.Instructor could have the students look up the refractive index of the materials if they want toadd a data search dimension to the lab.GemRefractometerMeasurements.docxHeffner 1/31/123
Appendix A: Under the Hood – How the Gem Refractometer Works(from http://gemologyproject.com/wiki/index.php?title Refractometer )Light (1) enters through the rear of the refractometer through an opening (1a) in (or before)which a yellow sodium filter can be placed. It then hits a mirror (2) which transmits the light tothe center of the hemicylinder (3). This hemicylinder is made of high refractive glass (usuallyN-LaSF by Schott with a refractive index of 1.88 at nD and a hardness of about 6.5 on Moh'sscale). At the boundary between the hemicylinder and the gemstone (4), the light will bepartially refracted inside the stone and partially reflected in the hemicylinder (see below on TotalInternal reflection). The reflected rays (5) will pass through a reading scale (6) and a lens (7) or aseries of lenses, depending on the type of refractometer.The reflected rays hit a mirror (8) which directs the light to the ocular (9) and then outside therefractometer to your eye (11). The ocular (9) can slide in and out for better focus and is usuallyaccompanied with a detachable polarizing filter (10).GemRefractometerMeasurements.docxHeffner 1/31/124
Appendix B: Sources for Gem Refractometer and SamplesGem Refractometers:The Gem Refractometer used to be a fairly expensive item (above 500) but more recently avariety of clones have been coming out of China, and you should not have to pay more than 100for a descent model. But you may have to shop around; use “gem refractometer” in your search.Both Amazon and Ebay are good places to find competitively priced products. There are severaldifferent versions out there. Some have internal light sources (single color, usually a yellowLED) while others use a detachable light source. Either seem fine. Most come with a smallbottle of the high index (RI) oil. A bottle of RI oil sold separately will cost about 20 so makesure your kit includes a bottle.I will list below the two recent purchases that I made, but you should have no problem findingsomething from a US supplier on Amazon.GEMOLOGICAL REFRACTOMETER, GEMSTONE TESTING INSTRUMENTOrdered on Amazon from sportswarehouse (Dec. 2011) for 97 with shipping but did not comewith the RI oil. Had to purchase that separately from them for 20 plus 5 shipping.The unit is nice and has an internal yellow LED as the light source.I also saw essentially the same item with oil from Ecrater (listed as a US seller) at 98 13.50Gem Refractometer w/ Built-in Light Source RI ometer-w-built-in-light-sourceMy most recent purchase was found on Ebay from Gemology Gemstone listed asGem Refractometer LED 1.81 RI OilIt ships from Australia but only paid 80 17 for S&H and it came within 10 days.It has an external yellow LED light source, which I prefer since I can now try different colorlight sources. So for me this is the best model at the best price.Sources for Sample Materials:The acrylate and PS samples are from window panes available at the local hardware store (e.g.,Home Depot) where these window materials are identified as follows:Plaskolite 8 in. x 10 in. Polystyrene Sheet, Model # 1S08104A, Store SKU # 628837, 1.97 ea.OPTIX 11 in. x 14 in. x .093 Acrylic Sheet, Model # MC-27, Store SKU # 241302, 3.98 ea.(Using the thicker 0.093” thick acrylate will help avoid confusion with the thinner PS pane)A very good, lower cost source for optics and optical glass is the Surplus Shed athttp://www.surplusshed.com/ All of the optical glass samples in this experiment came fromSurplus Shed and the items are listed below with current prices. F2 EQUILATERAL DISPERSION PRISM (L1885D) at 22.50 n 1.62High index, high dispersion flint glass sample (prism) – the most expensive sampleBK7 EQUILATERAL PRISM (L1851D) at 7.50BK7 is a more standard crown glass with n 1.71725MM DIA FUSED SILICA WINDOW (PM1078), 5 (closeout price)30MM DIAMETER LOW EXPANSION NOT ALUMINIZED MIRROR (PM1246), 3closeout price (LEBG is very similar to pyrex with an index close to fused silica)PM12213.5 mm th, opt window, BK7, D 20 mm 1.50 n 1.517GemRefractometerMeasurements.docxHeffner 1/31/125
Notes and References:1. Wikipedia provides a good orientation on the topic of refractive index, Snell’s law and thewavelength dependence or dispersion of common glasses at:http://en.wikipedia.org/wiki/Refractive index2. For an excellent description of the gem refractometer and how it works e RefractometerFor more detailed comparison info on various refractometers see:http://www.nordskip.com/refractometer.html3. For information on how to use a gem refractometer see one seller’s recommendation ions.htmor a slightly different version at How to Use a Gemological Refractometer eHow.comhttp://www.ehow.com/how 5002478 ere is also a You-tube demo on how to use gem refractometer titled: How to Use aRefractometer for Gem Testing Model FGR-003 at http://www.youtube.com/watch?v zi3PaHTKJ6U4. Edmonds Scientific has a very great summary table of the properties of optical glasses tics/optical-glass/?&viewallThe values in our table are obtained from this source. The data for plastics like acrylate and PSare available from the Wiki article on refractive index in reference 1 above, and also with moredetail and dispersion plots see http://refractiveindex.info/?group PLASTICS&material PS5. Wavelength reference for refractive index values used:For the Edmonds table the refractive index for optical glasses, nd, is specified at a wavelength of587.6nm (yellow, He d-line). Wikipedia suggests that “nD is the refractive index at theFraunhofer "D" line, the center of the yellow sodium double emission at 589.29 nmwavelength.”(Wiki). Of course, the difference in these two wavelengths is too small for us toargue about here.6. The chemical composition of the RI liquids commonly used are:1.79 - Saturated solution of sulphur and di-idiomethane1.81 - Saturated solution of sulphur, di-idiomethane and tetraidioethylene(from Gemology Project site mentioned in note 2 above)The Material Safety Data Sheet (msdi) for the 181 oil is available from the manufacturer at:http://www.cargille.com/pdf2/MSDS-Gem Refract.pdf7. For estimating the refractive index of sugar glass see the reference to the Feyman lecturecovering refractive index of sucrose already mentioned in notes from the Pfund Heffner 1/31/126
Extra stuff – probably won’t use:A very nice but much more technical review of refractive index measuring methods can be foundin the 2002 review paper:Shyam Singh, Refractive Index Measurement and its Applications, Physics Scripta., Vol. 65,167-180, 2002. Currently available online from MIT ctroscopy/Refractive Index/physscr 65 2 008.pdfAnother cool set of experiments related to index of refraction is available ion.pdfGemRefractometerMeasurements.docxHeffner 1/31/127
Refractometer to measure the refractive index of common glassy materials. To use the method the sample must have one flat surface. In this lab we provide a collection of glassy materials of different refractive index for the experimenter to measure and compare with known values. The Gem Refractometer can be purchased for approximately 100 and .