INVESTIGATING LIGHT-MATTER INTERACTIONS USING A MANUAL SPECTROMETER - A NEW CHM151 LAB Developed by E. Kwan, H. Ohorodnyk, I. Miller, A. Orozco, and A. Dhirani with assistance from F. Bures and J. Jackiewicz (electronics) as well as J. Ford and F. Shaw (machining). Department of Chemistry, University of Toronto.

Introduction Beer’s Law  Intensity of a beam of light decreases exponentially with the number of absorbing particles in the beam: I/I 0 = 10 -A where I is the final intensity, I 0 is the initial intensity, and A is the absorbance.  A =  c b, where  is the extinction coefficient, c is the concentration of the solution, and b is the path length. Goals of Project  Students explore the validity, limitations, and applications of Beer’s Law  Students are introduced to spectroscopy in a hands-on way

The Spectrometer Double Convex Lens Lamp Sample Holder Light Detector and I-V Converter Multimeter Diffraction pattern Photodiode Diffraction Grating  Light intensity measured by photodiode and multimeter  Detector position on rail determines wavelength detected Rails

Samples Filter ground up leaves soaked in methanol Extracted chlorophyll (1) Various filters (2) Copper (II) Sulfate Geraniums (3) Chlorophyll

Blue Filters: Background Removal  Red light intensity measured against number of filters  Data fit to y = y 0 + y 1 e -x/t, an exponential decay  y 0 is the background (stray light)

Blue Filters: Absorbance Plot  Absorbance calculated as –log[(I-y 0 )/(I 0 -y 0 )]  Absorbance increases linearly with numbers of filters  Slope is 0.31 (represents absorbance per blue filter)  Signal:noise ratio gets much worse

Green Filters: Absorbance Plot  Absorbance increases linearly but with a different slope,  Data measured at same red wavelength

Blue and Green Filters: Absorbance  Slope is now 0.45, statistically the same as the sum of => slopes add when filters combined  So absorbance is additive

Copper Sulfate Absorbance  Absorbance increases linearly with concentration  Graph can now be used to determine concentration of a solution by measuring its absorbance

 Green light transmitted  Red light absorbed and re-emitted (sideways also)  Some blue light converted to red light -> fluorescence (covered slit with blue filter and observed red light out) Spectroscopy of Chlorophyll Green Diffraction Maximum Green Reflection Chlorophyll appears red! Light Shield Transmitted Light Chlorophyll viewed from the side Chlorophyll viewed in normal light Special thanks to P.E. Trudeau and Y. Suganuma