Light and Color

1. Light and Color • light has both wave and particle characteristics • l = wavelength; n = frequency • c = speed of light = ln = 3.00 x 108 m/s • visible: l ~400 nm (violet) - ~750 nm (red) • energy in light comes in “packets” (particles?) called “photons” • Ephoton = hn = hc/l (h = Planck’s Constant) • different color = different l = different E per photon Light & Color

2. Light & Color https://en.wikipedia.org/wiki/Visible_spectrum

3. Light and Color (cont.) • white light = mixture of all different visible l’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 (l), but not ALL l’s in equal numbers. We see the “blended” (or sometimes just the most dominant) color Light & Color

4. Photons can be absorbed or emitted • 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) • 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” Light & Color

5. Recall: H atom emission (1st semester) Ephoton Light & Color

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”! Light & Color

7. Trichromatic Color Theory https://en.wikipedia.org/wiki/Cone_cell • (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 Light & 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 Light & Color

9. Light & Color

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 DE (energy difference or gap) between the orbitals, the greater the Ephoton absorbed • Different gaps yield different colors absorbed, and thus different colors perceived • Changing the DE in a metal complex (or other “dye” molecule) will change the color of the complex (or dye) Light & Color