Spectroscopy is all around us. The color in everything we see results from processes of absorption, reflection and sometimes emission. Absorption affects what we perceive as color by the removal of certain wavelengths of light from the spectrum. The color wheel shows us how we perceive color based on what is absorbed and therefore the complementary colors that are either reflected or reansmitted. Theories of the absorption and emission of light can explain these colors based on chemical properties. We can also explain why there is infrared and microabsorption based on molecular motions. Molecular vibrations can give rise to infrared absorption and rotations can give rise to microwave absorption. We can begin to understand spectroscopy in a quantitative way using the description of absorbance as a function of the concnetration of chemical compounds. This is known as Beer's law and it is described in the following video.
Fluorescence is a very important aspect of chemical technology today. Fluorescent dyes are used in cell biology studies, medicine and materials science to name just a few applications. Understanding why molecules emit light is a fascinating topic. Some molecules emit nearlly all of the light that absorb, while others are "black holes" simply absorbing light and not emitting any light at all that we can see. Of course, the release the energy as heat as they return to the ground state. This is a crucial aspect of fluorescence. There is always a competing non-radiative decay pathway. To understand the fluorescence yield we must understand the competing effects of a radiative and non-radiative pathway for return from an excited state to the ground state.
One application of spectroscopy is in the determination of concentration. We can also use spectroscopy to determine the limit of solubility of a compound. This quantitative application is described in the next section.
In coming chapters we will disucss acid/base chemistry in great detail. Titrations using colorimetric indicator dyes are an important part of the technology using in titrations. While sensitive analytical instrumentation for the detection of pH may be replacing some of the older methods there are still many applications for dyes and detection using color that are based on the same methods used for many years to detect the "end point" in a titration.
