1. 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

2. 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

3. 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

4. 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

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

6. 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

7. Trichromatic Color Theory
CHM 122
Week 12, I&II
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
Light & Color

8. 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

9. Light & Color

10. 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