1. Visible Spectroscopy Electromagnetic Radiation: Light & Color

2. is propagated at the speed of lightis propagated at the speed of light has properties of particles and waveshas properties of particles and waves the energy of a photon is proportional to its frequencythe energy of a photon is proportional to its frequency Electromagnetic Radiation

3. Electromagnetic Radiation http://chemistry.beloit.edu/Stars/EMSpectrum/index.html http://chemistry.beloit.edu/Stars/EMSpectrum/index.html

4. Cosmic rays g Rays X-rays Ultraviolet light Visible light Infrared radiation Microwaves Radio waves Cosmic rays g Rays X-rays Ultraviolet light Visible light Infrared radiation Microwaves Radio waves Energy Electromagnetic Radiation

5. Longer Wavelength ( ) Shorter Wavelength ( ) Higher Frequency ( ) Lower Frequency ( ) Higher Energy (E) Lower Energy (E) UltravioletInfrared Electromagnetic Radiation

6. 400 nm 750 nm Visible Light Longer Wavelength ( ) Shorter Wavelength ( ) Higher Frequency ( ) Lower Frequency ( ) Higher Energy (E) Lower Energy (E) Electromagnetic Radiation

7. Visible Light & Color Sir Isaac Newton (1704) used a prism to show that sunlight was composed of light with all colors in the rainbow. He defined it as the spectrum.

8. The Visible Spectrum and Color

9. Black and White vs. Color Light of all frequencies is white light, eg. sunlight Black is the absence of light, not color. Color is light of one or more wave lengths but not all. Candlelight lacks high frequencies. It emits yellowish light. Incandescent light emits light at all visible frequencies, but is richer towards the low frequencies and hence enhances the reds. Fluorescent light is richer in high frequencies and enhances blues.

10. The Retina & Photoreceptive Cells

11. Perception of Color Humans can distinguish hundreds of thousands of different colors Humans have 3 types of receptors –red (peak response at wavelength = 580nm) –green (peak response at wavelength = 545nm) –blue (peak response at wavelength = 440nm) Red, green, and blue are the primary colors. Light containing equal intensities off all three appears white. Complementary colors are magenta (green), yellow (blue), and cyan (red) http://chemconnections.llnl.gov/organic/Chem227/227assign-06.html#vision

12. Perception of Color Light containing equal intensities of red, green, and blue appears white. Red paint reflects red and absorbs the other colors. Paint and dyes contain tiny solid particles of some pigments, they usually reflect a wide rage of frequencies (mixture of colors) and absorb the rest. –Cyan pigments absorb red –Blue paint reflects blue, violet, green It absorbs red, orange, yellow. –Yellow paint reflects, red, orange, yellow, green. It absorbs blue, violet –When blue and yellow are mixed, they reflect only green

13. Color & Atoms When atoms are excited to higher energy levels from a ground state they emit the energy that was absorbed. –Neon gas produces a brilliant red –Mercury vapors produces violet –Helium produces pink The light emitted from each different element produces different light intensities versus the light frequency.

14. Gaps between electron energy levels correspond to wavelengths; between 200 and 800 nm (Ultraviolet-Visible) Transitions between energy states  E = h  E = h

15. Absorption and Emission of Light Energy AbsorptionEmission (Excitation from ground state or from an excited state to a higher state) (Dropping from an excited state to ground state or lower state)

16. Spectrophotometer

17. Attenuation of Light

18. Transmittance vs. Absorbance Transmittance scale is linear Absorbance scale is exponential

19. The Spectrum The spectrometer measures the intensity of a reference beam (P o =I r ) and the intensity of a beam through a sample (P=I s ). Absorbance is the log of the ratio Graph is absorbance vs. wavelength.

20. Beer’s Law Graph is absorbance vs. concentration. Beer’s Law: A =  cl; A =abc  (a) is the molar absorptivity, c is the sample concentration in moles per liter, and l (b) is the length of the light path in centimeters. C

21. Beer’s LawA = abc Path Length Dependence, b Readout Absorbance 0.82 Source Detector

22. Beer’s LawA = abc Path Length Dependence, b Readout Absorbance 0.62 Source Detector b Sample

23. Beer’s LawA = abc Concentration Dependence, c Readout Absorbance 0.82 Source Detector

24. Beer’s LawA = abc Concentration Dependence, c Readout Absorbance 0.62 Source Detector b Sample

25. Beer’s LawA = abc Concentration Dependence, c Readout Absorbance 0.42 Source Detector b Sample

26. Beer’s LawA = abc Wavelength Dependence, a Readout Absorbance 0.82 Source Detector

27. Beer’s LawA = abc Wavelength Dependence, a Readout Absorbance 0.30 Source Detector b

28. Beer’s LawA = abc Wavelength Dependence, a Readout Absorbance 0.80 Source Detector b

29. What is the manganese concentration in a sample that has an absorbance of 0.658, a path length of 1.50cm and a molar absorptivity,  5.85 x 10 3 L/mol*cm. A =  b c Prelab Calculation

30. What is the manganese concentration in a sample that has an absorbance of 0.658, a path length of 1.50cm and a molar absorptivity,  5.85 x 10 3 L/mol*cm. A =  b c 7.50 x 10 -5 M MnO 4 Prelab Calculation