Introduction: Laser Refractometer

In microscopy and other sciences, it is often desirable to know the index of refraction ( RI ) of diaphanous liquids. In a well equipped laboratory, the acceptable instrument for RI measuring is an Abbe refractometer, but the cost of these is well beyond the budget constraints of most hobbyists. Fortunately thither is a simple crushed cost disjunctive, for transparent liquids at least. The apparatus fundamentally consists of a vasiform equilateral triangular shabu prism which is full with the test liquid. A narrow shaft of light from a laser pointer is oriented through the apex of the prism, and the fish finished which the light electron beam is deviated is measured on a protractor surmount. This is the principle of the optical prism refractometer.

It can be shown that when the deviated angle is a stripped-down value, the refractive index of the molten in the prism can be calculated from the formula RI = 2 sin ½ ( d + 60 ) , where d is the stripped deviation angle of the light radiate. The lineage of this formula can be found in nearly physical science text books ( for example, "Physics Part with I &A; II" , Resnik & Halliday ( 1966 ) p 1018 ). Fig 1 illustrates the itinerary of the beam through the prism. The minimum deviation occurs when the ray of light passes done the prism parallel to the base

Supplies

3 x glass microscope slides 75 mm x 25 mm

Clear Silicone polymer Sealer ( or other suitable sealant : see at a lower place )

1 x empty CD-R container

Clear Acrylic packaging material

Double sided foam adherent record

1 x Protractor exfoliation

Rigid Polyvinyl chloride tube, 75 mm x 20 mm diameter

MDF Display panel, 250 mm x 180 mm x 18 mm approx

7 x 3 mm auto screws

Aluminium bracket ( approx 15 millimeter x 100 mm x 1 mm deep )

1 x 1 mW Optical maser Pointer

Step 1: Grammatical construction

The construction of a prism refractometer is relatively simple. The optical prism cell can live made from 25 millimetre square pieces of glass microscope slide which are cemented together at the edges and affixed happening another slide. Silicone sealer is right for use with nearly liquids, only separate sealants can represent used provided that they are inert to the test liquid. To ensure that the angles of the electric cell are as near every bit mathematical to 60 degrees, an equilateral triangular template can cost successful from stiff card and the cell collective around it, exploitation endless modelling pins to hold the sides in place while the sealant cures. Care of necessity to be taken over to keep the sealer confined to the joint surfaces only. If some is accidentally smeared on the faces of the cell, it can exist carefully far with a razor blade after curing. The stages of the construction of a prism cell are shown in Al-Jama'a al-Islamiyyah al-Muqatilah bi-Libya 2..

Footprint 2: Rotating Table

One time the mobile phone has been made, it needs to be mounted happening a rotating defer. An easy means of doing this is to obtain the base section of a blank Compact disc recordable container and uncomparable of the clear plastic spacers which came in the CD pack. The central spindle is trim back remove about 2.5 mm above the base and the clear spacer disc is slipped over this unmindful axle. The slide with the methamphetamine prism is then geosynchronous to the spacer phonograph record with treble sided foam tape, thus that it rightful clears the central spindle. The prism cell should then revolve around freely about the spindle when the plastic spacer is inverted. If the spindle is too high, data file a small number hit until the prism cell and slide do non touch IT.[See Fig 3 ]

Next you will need few set clear acrylic packaging material approximately 0.5 mm in thickness. From this, the index number build up is cut back exploitation the pattern in FIG 4. ( If the CD container you have is different, you may have to make some alterations to the pattern. ) The vertical indicator batting order butt be glued to a short "L" section as illustrated above.

Another vertical index tab is required for the aught check of the refractometer. This can represent made in the Sami way as the moveable exponent subdivision, but it does not need the central ring as it will equal fixed in set back. A pattern for this start is shown in Fig 5 supra.

Abuse 3: Protractor Graduated table

A protractor scale now of necessity to be made, which is placed underneath the index arm and the shiny disc holding the optical prism, and glued to the base of the CD-R container. A photocopy of a 360 level protractor was put-upon in the example illustrated, merely only one quadrant of the scale is actually needed. It is a good idea to clear laminate the protractor musical scale if you can, this wish preserve the markings indefinitely. A 40 mm diam hole is barge in the centre to fit finished the radix of the CD-R container. The zero index lozenge is glued in position to coincide with the cipher mark on the protractor scale. Ensure that the semi-climbing indicator line is on the nose aligned with the centre of the protractor and the zero degree note.. The index arm is set on top of the protractor scale and underneath the clear spacer disc carrying the optical prism jail cell. It should rotate freely without causing the spacer disc to move with it. The complete rotating table assembly is attached to a MDF base panel. The components are assembled as shown in Fig 6.

Step 4: Optical maser Cursor

The incoming step is to mount the laser cursor ( only a 1 mW pointer should live used – lasers are dangerous to your vision ) so that the beam passes exactly above the centre of the rotating table, and is aligned on the vertical mark of the zero index tab. A 75 mm length of semirigid PVC tubing and an aluminium bracket is wont to wax the pointer on the base board. The height of the laser pointer must be such that the laser beam passes through the central section of the prism on the rotating table. The tube is drilled with two sets of 3 small holes spaced at 120 degrees to take the car thread screws, which wish hold the optical maser pointer in situation. The screws should be a tight fit in the holes, so that they cut a thread into the PVC when they are inserted. Another muddle is drilled above the pushbutton switch and a have a go at it is inserted therein so that the laser beam can Be turned on Beaver State off by turning the screw. The pose of the arrow is then adjusted with the screws to ordinate the glow with the zilch mark. Make up careful to protect your eyes from the laser beam when it is switched along. Fig 7 shows the completed refractometer in use.

Step 5: RI Measurement

Once everything is aright aligned, the prism can glucinium filled with the test watery, exploitation a pasteur pipette. All but 5 ml of taste is compulsory. If the swimming is volatile, place a 25 mm right-angled of slide glass on top to retard vaporisation. The optical prism and the clear plastic disc are then carefully rotated until a position is found where the departure angle of the laser beam has a stripped-down value. A magnifying lens wish be found useful Hera to read the tilt off the protractor weighing machine, again taking care not to look into the optical maser beam. Information technology is a honorable idea to switch off the laser while you are inspecting the protractor descale. The prism should then atomic number 4 rotated by 120 degrees so that the next corner is at the apex, and the minimum angle Reading affected for each apex. If your optical prism has been accurately ready-made with 60 stage angles, you should obtain the same minimum deviation angle for each of the 3 vertices of the optical prism. If they are slightly different, then just utilise the average value of the 3 readings, this will not introduce any earthshaking error.. The average rate of d is substituted into the rule RI = 2 wickedness ½ ( d + 60 ) to calculate the index of refraction, OR the RI value can make up read from the graph higher up.

Step 6: Appendix a : Sources of Error in R.I Measurements

Whatsoever comparisons were successful between R.I measurements made on this setup and a laboratory grade Abbe refractometer reading to +/- 0.0002. The results are tabulated in a higher place :

As can equal seen, the ordinary error is about 0.5 % . This would be adequate for most nonprofessional microscopy purposes. The sources of misplay can be identified in three main categories :

1 : Measurement of the Minimum Divagation Angle

The protractor scale can only be read to +/- 0.5 degrees, which corresponds to an uncertainty of around +/- 0.005 in the refractive index value. The accuracy of the tip over measurement could be improved by projecting the laser beam at right angles onto a wall at a distance of a few metres and measuring the stripped-down lateral displacement of the beam in the flat flat. ( imag in a higher place )

The minimum deviation angle commode then be calculated as d = arctan ( h / l ) , where h = lateral pass deracination and l = distance from the centre of the prism cell to the wall in. Forward l = 2000 mm and d = 30 degrees, and thus h = 1155 mm, with the laser pointer that I used, the diameter of the laser beam smirch along the wall is close to 3 mm. This corresponds to an uncertainty in d of about 1.5 arc minute, or +/- 0.0003 in the refractive index value, which is forthcoming the accuracy of laboratory instruments.

2 : Wavelength and Temperature Errors
Refractive Power decreases with increasing wavelength of the light source. The standard wavelength for R.I measurements is 589.3 nm, the wavelength of the sodium D line, whereas cheap red laser pointers produce a wavelength of 670 nm. For about organic liquids, this probably introduces an error of about 0.002 in the R.I value, the treasure at the Atomic number 11 D line being higher. If you backside obtain a yellowness-orangeness laser pointer, this leave give more accurate results, as these lasers have a wavelength of 583 nm.

Similarly, the R.I decreases with acceleratory temperature, for most substances this amounts to about – 0.0003 per degree Celsius. For best truth, the temperature of the sample needs to be controlled.

3 : Dimensional Irregularities in the Prism Cell
Variations in the thickness and perpendicularity of the chalk walls and the prism angles will cause errors in the R.I values obtained. A difference of +/- 5 degrees from the correct prism angles of 60 degrees will produce an error of about +/- 0.001 in the R.I value determined aside averaging the three d values. Hollow spyglass and formative prisms can follow obtained from commercial technological equipment suppliers, and these are probably more accurate than home made prisms.

Calibration of the Refractometer with Distilled Water

The best method of checking the refractometer's accuracy is to quantify the R.I of a try out of distilled ( or deionised ) water. At 670 New Mexico and 20 degrees Anders Celsius, the refractive exponent of water is 1.3317.

Abuse 7: Appendix B : R.I Value of Mixtures

In microscopy, it is oftentimes required to bring on a serial of liquids having known R.I values. It is sole necessary to obtain two liquids having legendary R.I values which span the range of interest, and a series of second R.I values can be competitive aside blending the two components. To a fairly good approximation, the R.I value of a mixture volition Be proportional to the book concentration of the two components., providing the two liquids do not interact to get prodigious volume dilation OR contraction. Thusly, given smooth A with R.I = a , and liquid B with R.I = b , the expected R.I value of any mixture is given by R.I = A a + B b , where A = volume fraction of liquid A and B = mass divide of watery B. ( Mention too that B = 1- A ). The graph above shows this relationship.

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