Light and Color light has both wave and particle characteristics CHM 122 Week 12, I&II 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

https://en.wikipedia.org/wiki/Visible_spectrum Figure: 05-03 Title: The electromagnetic spectrum Caption: Figure 5.3  The electromagnetic spectrum consists of a continuous range of wavelengths and frequencies, from radio waves at the low-frequency end to gamma rays at the high-frequency end. The familiar visible region accounts for only a small portion near the middle of the spectrum. Note that waves in the X-ray region have a length that is approximately the same as the diameter of an atom (10–10 m). Light & Color https://en.wikipedia.org/wiki/Visible_spectrum

CHM 122 Week 12, I&II 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

Photons can be absorbed or emitted CHM 122 Week 12, I&II 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

Recall: H atom emission (1st semester) CHM 122 Week 12, I&II Recall: H atom emission (1st semester) Ephoton Light & Color

Selective Absorption “Causes” Things to Appear Colored CHM 122 Week 12, I&II 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

Trichromatic Color Theory CHM 122 Week 12, I&II 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

Absorption Spectrum Plot of Absorbance (Abs) vs. Wavelength CHM 122 Week 12, I&II 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

Light & Color

Absorption at the Molecular Level CHM 122 Week 12, I&II 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