Classroom. An Easily Constructed Monochromator

Transcription

1 Classroom In this section of Resonance, we invite readers to pose questions likely to be raised in a classroom situation. We may suggest strategies for dealing with them, or invite responses, or both. Classroom is equally a forum for raising broader issues and sharing personal experiences and viewpoints on matters related to teaching and learning science. M Farooq Wahab Department of Mathematics & Basic Science, NED University of Engineering & Technology Karachi-75270, Pakistan farooq.w@gmail.com Keywords Monochromator, spectroscopy. An Easily Constructed Monochromator A functional monochromator model in a cardboard box, made from rotatable grating using a digital versatile disk is described to highlight the working principles of a UV-Vis monochromator. The light source is a bright white LED and the colored bands from the exit are displayed on the screen. The concept of absorption, complementary color, excitation and fluorescence, flame emission spectrum, and wavelength dependence of light scattering can be visually shown in this model. With a minor modification, one can use the same box to monitor flame emission spectrum of sodium and different light sources. As an optional activity, the same properties are observed on a scanning spectrophotometer. Most of us are familiar with spectrophotometers and perhaps have done experiments in college labs verifying and using the Beer s law, a law which relates absorbance of monochromatic light with concentration of a substance. This is an example of a vast field called spectrochemical analysis. Experiments with light are able to arouse considerable interest among students. Spectroscopy is one of the major scientific tools, which is helping to improve the quality of human life and has helped us know 996 RESONANCE October 2009

2 about atoms. Modern spectrochemical analyses, with their speed and sensitivity, help us know what traces of pollutants in our environment are ruining the health and life of human beings, by being able to detect toxic metals or organic pollutants as low as few parts per million. The heart of many modern instruments found in the chemistry laboratory, e.g., HPLC detectors, molecular or atomic spectrophotometers and spectrofluorometers is the monochromator. We usually encounter monochromators as a black box, while selecting wavelengths on a spectrophotometer. Recall the dial on the Spectronic with which we select wavelengths before noting absorbance of colored solutions. Monochromators are mechanical devices which selectively transfer a narrow band or color of light from a light source. The major fraction of the cost of a spectroscopy-based instrument is the monochromator. This implies that opening up a monochromator in a spectrophotometer to show its working to students is cost-wise formidable. For most of the students, the monochromator is a component which somehow selects wavelengths when rotating the dial on the instrument. This model will show that wavelength selection is rather easy. The high cost of the monochromator is due to high-precision optics and the high-quality reflection grating. In this article, the construction of a functional monochromator with easily available objects is described. The study of science should not be confined only to school or college laboratories but simple instructive experiments should be encouraged at home, because this path leads to studying science as a hobby. Though it is a very crude model and is not calibrated for wavelengths, it is functional enough to work with a table of corresponding colors and wavelengths and can be made in little time. Further, one can also use it in understanding a number of spectroscopy concepts such as absorption, excitation, scattering and flame emission. Many articles have appeared in the educational literature using a CD or DVD for teaching basic concepts of spectroscopy [1 3] but they require direct viewing of the CD and none involve movable grating and allow change of spectral on a paper screen. Also the concept of wavelength selection is not very clear in those articles. The activity in this article is an extension and improvement of the cardboard apparatus [4] used by the author to demonstrate the principles of fluorescence spectroscopy. The basic theme that one knows how to solve an instrument problem when one knows its working principles can best be shown by a dialogue between G N Lewis and his graduate student Michael Kasha, who was to become an eminent photochemist. Kasha was perplexed that his dye solutions were unexpectedly showing prominent absorption curves in the infrared using a Beckmann UV-Vis spectrophotometer. Set the monochromator at 1000 nm, Lewis told him after opening up a spectrophotometer and looking at the exit slit, Aha, I am the first man to see the infrared. And it is green! said Lewis, one of the most important contributors to the world RESONANCE October

3 of chemistry and spectroscopy [5]. It turned out that the monochromator was obviously misbehaving by throwing green light at the exit slit when set at the infrared wavelength. Lewis wanted to teach a lesson be cautious with instrument performance. The principle of a monochromator is quite simple. We only need a mirror (here we used a lens) to collimate the incomingbeamof lightfromthe slit, and thena diffractingelement the reflection grating is used to disperse the different wavelengths in space. A focusing mirror, then focuses the desired diffracted wavelength onto the exit slit. In this way, rotation of the grating makes different wavelengths fall on the exit slit. The design we will use here does not involve a focusing element although that can be achieved by placing another magnifying glass near the exit slit. In an automatic system, a motor drives the grating, while manual monochromators require you to turn the grating to select a wavelength leaving the exit slit. The rotation of the grating is calibrated in wavelengths. Here, we will be dealing with colors rather than wavelengths the colors can then be correlated with wavelengths. Details of the working principles of the monochromators can be found elsewhere [6]. Materials For the monochromator: A cardboard box about 28 cm long, 18 cm wide and 10 cm depth with a cover, a black tape, a bright white LED, a magnifying glass (focal length mm), a laminated DVD (unused Sony DVD-R), a 15 x 15 cm white cardboard, glue, four razor blades, Plasticine, a round glass cup (~ 200 ml). Preparation of the Solutions (a) For absorption experiments, a few crystals of KMnO 4 are added to 200 ml distilled water to make it purple (approximate concentration 5 x10-5 M or less). (b) For fluorescence experiments, 200 ml of fluorescent ink is made as follows: The fluorescent yellow highlighter marker (Schneider Job Universal-Germany) tip is dipped several times in distilled water until it acquires a very light green fluorescent color. One may substitute fluorescein. (c) For scattering experiments, a drop of milk is added to 200 ml of water that makes it turbid but effectively transparent. (d) For flame emission experiments, a Bunsen burner is needed, and a 1 cm diameter wheat dough ball is made which is rolled in NaCl so that large amount of crystals stick on the dough, and a long and thick iron nail is then half inserted into the ball. The experiments must be performed in a dark room. 998 RESONANCE October 2009

4 Construction of the Monochromator On the cardboard box, make a vertical entrance slit S1, 2 cm high and 1 cm wide, in the center of the shorter side as shown in Figure 1. Cut out another vertical slit S2, 2 cm high and 0.5 cm wide, on the longer side of the box, about 12 cm from the point D. Paste razor blades over the slits. Now place a bright white light emitting diode (LED), - 5 cm from the entrance slit. Holding the magnifying glass upside down (Figure 2), move it slowly so that a large bright white spot is formed at the center of the side BC. In this way, it will illuminate the whole grating. The optimum position is the point where the LED is at the focus of the magnifying glass. It is important to point out here that in real monochromators, the collimating mirror or lens has its focus situated at the slit to make incoming rays parallel. Here, we have a large entrance slit, so we have kept the lens at the focus of the LED rather than at the entrance slit. Typical magnifying glasses have focal lengths of 15 to 20 cm, i.e., the distance between the light source and the lens falls within this range. When the optimum positions have been found, fix the magnifying glass with the help of a support or Plasticine. In order to make a grating use a laminated DVD, cut out a slice measuring approximately 2 x 2 cm from the outer edge with scissors or small-sized plant shears. Separate the two layers. This reveals a shiny surface which looks like a real reflection grating. One should be aware that the grooves on a DVD are circular. Since we cut a small size from the outer most edge of DVD, they can be considered linear. Glue this grating to a ball point pen P at the same height as the position of the light spot formed on the side BC of the cardboard box. However, while pasting the DVD slice on the pen, the outer edge (the round edge) of the DVD slice must be on the left-hand side. Figure 1 (left). A schematic diagram of the DVD monochromator. The glass cup (dotted circles) is placed for absorption experiment at position 1. The second position 2 is used for fluorescence experiments as excitation monochromator. Figure 2 (right). The uncovered DVD monochromator. The magnifying glass and the pen on which the grating is pasted are supported by Plasticine. The images of different colors are seen on the white screen. A B LED 1 S1 M G P D 2 S2 C S RESONANCE October

5 This improves the spectral display. Place the pen at some distance from the magnifying glass. Rotation of the pen makes bright spectral display of rainbow on the sides of the box. Note that when the slice is pasted as described, clockwise rotation makes better display at the exit slit S2. Adjust the position of the pen where intense colored images of the slit, exit at S2. Attach a white screen S, 5 cm from the point D. While rotating the pen, one should see a white light rectangle (zero order), i.e., reflected light, and further rotation produce images of different colors on the screen. After these adjustments have been made, make corresponding holes, for the pen and the holder of the magnifying glass, in the cover of the cardboard box. Pass the holder of the magnifying glass and the pen through the cover. The monochromator is ready for experimentation. When the monochromator is used for observing the flame emission of sodium, the magnifying glass should be removed, and the DVD slice rotated anticlockwise. In this way, the lines appear straight; on clockwise rotation, lines appear curved. This is because the grooves on the DVD are circular. Experiments with the Monochromator Absorption experiments: We first show how to use this monochromator to demonstrate the idea of absorption spectrum and the concept of complementary colors. Place a glass filled with water at position 1. Turn on the LED and while slowly rotating the pen, observe the colors appearing on screen S. Then place a purple solution of dilute KMnO 4 at position 1. Turn on the LED and note the colors appearing on the screen. Note what colors are missing. You will notice that when potassium permanganate solution was placed, green color did not appear on the white screen, whereas the rest of the colors (present in the white LED spectrum) are clearly visible. This is an example of absorption spectrum and one can relate the apparent color of the solution and missing colors in the spectrum. Similarly, an orange food dye shows a transmitted spectrum having green, orange and red color. A chart of complementary and absorbed colors would be helpful for correlation [7]. Fluorescence experiments The same monochromator is used as an excitation monochromator for a fluorescence experiment to show that the complementary color concept is not true for fluorescent substances when they are in dilute solution. One can also show the concept of Stokes shift rather easily. Stokes shift refers to the diffrence in absorbed wavelengths (colors) and emitted wavelengths (colors). For this experiment, the glass cup is placed at position 2. First, the experiment is repeated with water in the glass to see the colors present in the white LED and then, ink solution is filled in the glass. The ink should be present in low quantities so that it does not completely absorb the light. Note the grating also reflects zero order, which is simply the 1000 RESONANCE October 2009

6 reflected light, consisting of undispersed wavelengths. This white light also causes fluorescence and should be noted by seeing white light on the screen. Slowly rotate the pen and view the screen as well as the glass from the top. At a certain position, a green beam is visible in the glass. This is the fluorescence in the ink. Note what color is causing fluorescence in the ink. It appears that violet color on the screen is appearing. Note the color causing fluorescence and the color of fluorescence. Since, energy and wavelength are inversely related by the Planck s relation ( E hc / ); violet color corresponds to higher energy and shorter wavelength and green color of fluorescence is of lower energy and longer wavelengths. This is called Stokes shift in spectroscopy. Scattering Experiments: By placing a slightly turbid milk water solution at position 2, rotate the grating and observe from the top of the glass, very interesting observations are made. At a position when white light exits S2, a white beam is seen traveling in the solution. Further rotation, throws out violet and blue bands fromthe exit slit, and the solution still shows a dim ray traveling in the solution. By further movements, the white ray suddenly disappears from the turbid solution, i.e., when other colors are passing through the solution. This can be explained as follows: The turbid solution is scattering light; it is known that shorter wavelengths are scattered more strongly than longer wavelengths. Thus, when we pass violet/blue band we see a scattered white beam. The scattering decreases at fourth power of wavelength so, when we move towards longer wavelengths, scattering suddenly disappears. Flame Emission Experiments: Since sodium is easily excited in an ordinary flame, a simple arrangement can be used to see the emission spectrum of sodium to give an idea of emission of specific wavelength(s) by atoms when excited in flame. When we introduce sodium chloride in a flame, the flame immediately becomes yellow for a very brief period. This is not satisfactory for this experiment. A simple technique will provide sodium emission spectrum for more than 15 minutes in a flame. A paste of wheat flour and table salt is kneaded into a small ball of about 1 cm in diameter and stuck onto a metallic wire. If this wire-ball system is introduced in the flame with the help of a holder or tongs, it will continue to produce the yellow light for quite sometime (the dough initially burns black but continues to provide sodium emission). Now, if this flame is observed through this spectroscope, by placing the spectroscope on a support in such a way that the slit is aligned with the flame, one will see a very faint continuous spectrum due to the flame; slight rotation of the grating shows a very intense yellow line characteristic of sodium. It is interesting to see that this is the only line which is visible in the spectrum. This line is very famous and is called the sodium-d line (589 nm). The instructor can point out that sodium atoms are actually causing the yellow emission (rather than sodium chloride) and discreteness of the electronic energy levels. According to Strobel [8], the term originated RESONANCE October

7 because the discrete wavelengths emitted by free atoms appear as images of the entrance slit in dispersive spectrometers, i.e., as bright rectangular lines. Perhaps, if the early spectroscopists employed small round holes rather than rectangular slits, we would have point spectra rather than line spectra. Concept of Throughput: The students can further experiment by decreasing the size of the slits and note a very important concept. This is the concept of throughput, i.e., the energy that enters and leaves the optical system. Larger the size of the slit, greater is the energy that is able to pass through the monochromator, but the resolution decreases. Narrow slits increase resolution but decrease the energy. This is a problem with sources with limited intensity and for such sources, resolution and slit-widths must be traded off. In order to try this idea, the experimenter reduces the width of the slits S1 and S2 by half and notes the intensity of fluorescence of the ink which decreases considerably when the slit width is reduced. Conclusion A functional monochromator was made from easily available objects to give an idea of the working of real monochromators. Five simple experiments, which are integral to general spectroscopy, are described. One can easily do all the experiments with a minimum cost. The same experiments are compared with a monochromator of a scanning spectrophotometer. Suggested Reading [1] Joel Tellinghuisen, Exploring the Diffraction Grating Using a He-Ne Laser and a CD-ROM, Journal of Chemical Education, Vol.79, pp , 2002 [2] JCE Editorial Staff. CD Light: An Introduction to Spectroscopy, Journal of Chemical Education, Vol.75, pp.1568a-1569a, [3] F Wakabayashi, K Hamada, K Sone, CD-ROM Spectroscope, A Simple and Inexpensive Tool for Classroom Demonstrations on Chemical Spectroscopy, Journal of Chemical Education, Vol.75, pp , [4] M F Wahab, Fluorescence Spectroscopy: in a Shoebox, Journal of Chemical Education, Vol.84, pp , [5] Micheal Kasha, The Triplet State. Journal of Chemical Education, Vol.61, p.204, [6] Jose L Guiñon, J Garcia-Anton, Experimental study of monochromators in UV-Vis and IR spectrophotometers. Journal of Chemical Education, Vol.69, pp.77 78, [7] Daniel Harris, Quantitative Chemical Analysis, 3rd ed. Freeman and Company, p.506, [8] H A Strobel and W R Hienman, Chemical Instrumentation: A Systematic Approach, pp , John and Wiley, RESONANCE October 2009