1. Light and Color light has both wave and particle characteristics – = wavelength; = frequency –c = speed of light =  = 3.00 x 10 8 m/s –visible: ~400 nm (violet)  ~750 nm (red) –energy in light comes in “packets” (particles?) called “photons” E photon = h  = hc/  (h = Planck’s Constant) –different color = different = different E per photon 1 Light & Color

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3. Light and Color (cont.) white light = mixture of all different visible ’s of light (the whole rainbow) –all different kinds of photons, in roughly the same numbers, reach our eyes monochromatic light = light of one wavelength (color) –atomic emission line, laser pointer light most colored light is polychromatic: a mixture of different colors ( ), but not ALL ’s in equal numbers. We see the “blended” (or sometimes just the most dominant) color 3Light & Color

4. Photons can be absorbed or emitted Absorption of a photon: an atom or molecule absorbs the energy of a photon and the photon “disappears” – PE goes up (at least initially) –“endothermic” Emission of a photon: if energy is released by an atom AS a photon –PE of atom goes down; energy “turned into” a photon (creates one) Light & Color 4

5. 5 Figure 7.7 A Change Between Two Discrete Energy Levels Emits a Photon of Light

6. Selective Absorption “Causes” Things to Appear Colored If white light shines on (or through) a substance (or solution), and only certain kinds of photons (i.e., colors of light) are absorbed, the light that makes it to our eyes will be perceived as having a “color”! 6Light & Color

7. Trichromatic Color Theory (see sheet) All colors can be thought of as different mixtures of three primary colors: R, G, B –our eyes have receptor molecules that are sensitive to the absorption of these three colors, basically In equal amounts: –R + B = M (magenta) –B + G = C (cyan) –R + G = Y (yellow) (Seems odd!) In different amounts, all different colors –Colors on Computer! Projection TV’s 7Light & Color

8. Absorption Spectrum Plot of Absorbance (Abs) vs. Wavelength The larger the “absorbance”, the greater the relative number of photons (of that wavelength) being absorbed by the sample. –If primarily R is being absorbed, G and B are transmitted (or reflected), and we see B + G = C –If primarily G is being absorbed, R and B are transmitted and we see R + B = M –If primarily B is being absorbed, R and G are transmitted, and we see R + G = ____ See handout with spectra 8Light & Color

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10. Absorption at the Molecular Level Absorption of one photon of visible light corresponds to the excitation of one electron from a lower energy orbital to a higher energy one The bigger the  E (energy difference or gap) between the orbitals, the greater the E photon absorbed Different gaps yield different colors absorbed, and thus different colors perceived Changing the  E in a metal complex (or other “dye” molecule) will change the color of the complex (or dye) 10Light & Color