equal to some energy level difference in the atom or molecule. In figure 8.2 the

red light does not have enough energy to excite the electron to the higher energy

level. The blue light has too much energy and therefore does not excite the

electron to the higher energy level. But the green light has exactly the right

amount of energy to excite the electron to the higher energy level. This molecule

will absorb green light but will not absorb red or blue light.

Ultraviolet-Visible Absorbance Spectroscopy

Many different wavelengths (IR,

microwave, x-ray, etc.) of light can

be used in absorbance

spectroscopy. One of the most

widely used wavelength ranges for

absorbance spectroscopy is light in

the ultraviolet and visible (UV-Vis)

regions of the electromagnetic

spectrum. The UV-Vis region has

energies of photons that are often

similar to energies required to

promote an electron from one

energy level to another energy

level. In UV-Vis absorption

spectroscopy we measure how

much light passes through a sample

at all of the different wavelengths.

If we look at a liquid sample and

observe it to have a specific color

with our eye, the light that is being

absorbed by the sample is the

complement of that color on a color

wheel. Using figure 8.4 as a guide,

we can demonstrate how this

works. A solution that appears

yellow to our eye, for example, is absorbing the complementary color purple. We

would expect a yellow solution to show strong absorbance for photons near 435

nanometers.

61

Figure 8.3: Derived from "File:Light wave harmonic

diagram.svg" by Rubber Duck (☮ • ✍) is licensed

under CC BY-SA 3.0.

Figure 8.4: "File:Color wheel wavelengths.png"

by Tem5psu is licensed under CC BY-SA 4.0.