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Enabled Demonstration of Optical Rotation 7 Patrick I Page 1 of 11 Submitted to the Journal of Chemical Education 1 2 3 4 Is that a polarimeter in your pocket? A zero-cost, technology- 5 6 enabled demonstration of optical rotation 7 Patrick I. T. Thomson* 8 9 University of Strathclyde, Department of Pure & Applied hemistry, 295 athedral Street, Glasgow, 10 5 G1 1(L 11 12 13 ABSTRA T 14 A polarimeter can be constructed using a document camera as a visualizer and a standard 15 16 smartphone screen as a source of polarized light. A pair of ,D glasses can be used as a polarizing filter 17 18 and the optical rotation angle of li-uids can be measured using any free compass app. onse-uently, a 19 20 10 demonstration of optical rotation can be conducted with no preparation time or specialized e-uipment. 21 22 GRAPHICAL ABSTRA T 23 24 25 26 27 28 29 30 31 32 33 34 35 %EY(ORDS 36 Audience. First0Year Undergraduate 37 38 15 Domain. Demonstration, 2rganic hemistry 39 40 Pedagogy. 3ands0on Learning, 4anipulatives 41 42 Topic. hirality, 2ptical Activity 43 44 45 46 Practical demonstrations, conducted by a lecturer in front of an audience, are as old as science 47 48 20 teaching itself.1 Demonstrations involving the transmission of light, for e5ample solution0state color 49 50 changes in chemistry, have been conveniently presented to large audiences using overhead pro6ection 51 52 technology for many decades.2,, 2ne area that this has been applied to with great success is that of 53 54 optical rotation. The phenomenon of the rotation of plane polarized light by molecules that e5hibit 55 56 57 58 59 Journal of Chemical Education 3/6/18 Page 1 of 11 60 ACS Paragon Plus Environment Submitted to the Journal of Chemical Education Page 2 of 11 1 2 3 chirality.,015 Such demonstrations can be readily converted into hands0on e5periments,17021 but the 4 5 25 re-uirement for specialized or self0constructed e-uipment presents a barrier to entry. 6 7 The measurement of the plane of polarized light re-uires the use of an instrument called a 8 9 polarimeter. A polarimeter consists of a source of monochromatic light, a polarizing filter, an area 10 11 where this light can pass through a sample, a second rotatable polarizing filter 8the analyzer9, and a 12 13 detector to measure the intensity of the resulting light and conse-uently the angle of rotation. In the 14 15 30 absence of a sample, the light transmission will be ma5imized when the polarizers are aligned, and 16 17 minimized when the polarizers are perpendicular to each other. hiral molecules rotate the plane of 18 19 polarized light by an angle dependent on temperature, concentration, path length, wavelength of light, 20 21 and the specific rotation of the sample. This angle can be measured by rotating the analyzer to 22 23 minimize or ma5imize the transmitted light, then converted to a specific rotation. 24 25 35 Literature values for specific rotation are cited at a particular temperature and monochromatic 26 27 wavelength, most commonly 589nm. Specific rotation is different at different wavelengths, so for this 28 29 reason, measurements taken in the lecture theatre using white light sources also e5hibit optical 30 5 31 rotatory dispersion 8colored fringes of light on either side of the angle of minimum transmission9. 32 33 Demonstrations, then, are usually limited to -ualitative observations, such as the e-ual and opposite 34 40 optical rotation of enantiomers, or the modification of chiral activity during a solution0phase 35 36 reaction.7,22 37 38 Demonstrations also re-uire the use of polarizing film, and e-uipment to hold and rotate this film. For 39 40 these demonstrations, polarizing film has been sourced from certain types of consumer electronics,17 41 42 certain kinds of sunglasses,2,025 or purchased from scientific laboratory suppliers. The holder can be 43 44 45 constructed from ine5pensive materials,17 but combined time and effort remains a barrier to entry for 45 46 a simple lecture demonstration. Additionally, the traditional overhead pro6ector typically contains a 47 48 high0powered incandescent light source which can also pose a fire hazard when using flammable 49 50 solvents. 51 52 The current demonstration is one which can be assembled e5tremely -uickly from entirely 53 54 50 household items, namely a smartphone and a pair of ,D glasses. It can be displayed using the now0 55 56 ubi-uitous lecture support, the document camera. And it can be used to measure household 57 58 59 Journal of Chemical Education 3/6/18 Page 2 of 11 60 ACS Paragon Plus Environment Page 3 of 11 Submitted to the Journal of Chemical Education 1 2 3 substances. solutions of sugar. Demonstrations of chemical principles created from mundane items 4 5 are more engaging for students, and reduce the perceived abstract nature of the principle being 6 7 demonstrated.27 A diagram of the assembly of the polarimeter is shown below 8Figure 19, where the 8 9 55 smartphone acts as both source of light and a rotatable polarizing filter. The ,D glasses act as the 10 11 second polarizing filter, and the document camera acts as a method of measuring and displaying the 12 13 angle of rotation. A video of the demonstration being used in a lecture is available in the Supporting 14 15 Information. 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 60 Figure 1: chematic of the polarimeter 31 OMPO,E,TS O- THE POLARIMETER 32 To run this demonstration, the following materials and e-uipment are re-uired. A source of 33 34 polarized light 8usually a smartphone9, a pair of ,D glasses, a document camera, a chiral li-uid or 35 36 solution of a chiral molecule 8usually sucrose9, and a way of measuring angles 8a protractor, or a 37 38 65 compass application installed on the smartphone9. 39 40 41 Light source: the smartphone 42 The first item of technology used in this demonstration is the smartphone. Smartphones are a 43 mainstay of the student pocket and higher education practitioners and researchers have been -uick to 44 45 capitalize on this, with such advances as in0lecture polling, student video production, and lab 46 47 70 notebooks.270,0 3owever, as well as a platform for digital and visual content, the smartphone has 48 49 physical hardware capable of specific interactions with the chemical world. For e5ample, the camera 50 51 has been used as a colorimeter for spectroscopy, and the audio output has been used to control an 52 53 electroformation chamber.,1,,2 2f relevance to the current research, mobile phone screens are 54 55 polarized light sources.24 A li-uid crystal display 8L D9 emits polarized light as a necessary by0product 56 57 58 59 Journal of Chemical Education 3/6/18 Page 3 of 11 60 ACS Paragon Plus Environment Submitted to the Journal of Chemical Education Page 4 of 11 1 2 3 75 of its technology,,,,,4 but polarizing film is a component of more modern organic light0emitting diode 4 5 82LED9 displays as well in order to eliminate internal reflection and glare.,5 It seems, therefore, that 6 7 pocket polarized light sources are everywhere, and here to stay, although some newer models of phone 8 9 screens use circular polarizing filters which need a modification to the current demonstration 8see 10 11 Supporting Information9. Particular acknowledgement at this point is given to the recent prior work of 12 13 80 Schwartz et. al.24 who were the first to use the same L D as a source of light and a polarizing filter 14 15 simultaneously. A tablet computer or laptop screen can also be used if no suitable smartphone is 16 17 available. 18 19 Polari%ing filter: the 3D glasses 20 The second item of technology used in this demonstration is a pair of stereoscopic glasses for use 21 22 85 in ,D cinema. There are two widespread types of ,D cinema technology at the time of writing, 23 24 corresponding to two types of polarization. linear polarization 8used in ,D I4A(? screens9 and circular 25 26 polarization 8used in other types of ,D cinema9. I4A(?0compatible ,D glasses can conse-uently be 27 28 used as linear polarizers without modification, and can be purchased cheaply or obtained after 29 30 watching a film.,7 The circular polarizers found in “AealD0,DB glasses can be converted to linear 31 32 90 polarizing film by peeling off the inside layer with a razor blade, or potentially used as0is as part of a 33 34 demonstration of circular dichroism. If you have ,D glasses of unknown technology, they can be worn 35 36 while e5amining a suitable smartphone. The screen will fade to 100C black on rotation only when 37 38 viewed through linear polarizers. ertain kinds of anti0glare sunglasses have also been used in prior 39 40 demonstrations.2,025 41 42 95 Analy%er: the document camera 43 The third item of technology used in this demonstration is the visualizer, or document camera. 44 45 This relatively new article of technology has fast become an essential part of the education repertoire 46 47 when paired with a digital pro6ector. In chemical education, it has been used to demonstrate chemical 48 49 reactions,,7 color changes, and even optical rotation.24 3istorically, an overhead pro6ector 823P9 was 50 51 100 used for the same purpose,2 and optical rotation was one of the few demonstrations that was easier 52 53 with the older technology, owing to the 23PDs built0in light source.,8 3owever, the document camera is 54 55 a more versatile tool, as it can record and pro6ect non0transparent demonstrations onto screens of 56 57 58 59 Journal of Chemical Education 3/6/18 Page 4 of 11 60 ACS Paragon Plus Environment Page 5 of 11 Submitted to the Journal of Chemical Education 1 2 3 arbitrary size.
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