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M-3
There are no discontinuities in the properties of radiation as we pass from one region of the
spectrum to another. Note that visible light (radiation which we can detect using our eyes),
represents only a very small fraction of the electromagnetic spectrum from ca. 380 nm to
780 nm.
B.
Absorption of Electromagnetic Radiation by Molecules
The study of the interaction between matter and electromagnetic radiation is referred to as
spectroscopy. Spectroscopy may be used both qualitatively to determine if a substance is present
in a solution and quantitatively to determine the concentration of that substance in solution.
When a molecule absorbs a photon of energy, the energy of the molecule increases. In chemical
terms, we say that the molecule has been promoted to an excited state. If a molecule emits a
photon, its energy is lowered. The lowest energy state of a molecule is called the ground state.
Molecules do not absorb all wavelengths of light equally. In fact, molecules only absorb
electromagnetic radiation of specific wavelengths. Structural information about a molecule can
be obtained by studying the absorption of electromagnetic radiation as a function of wavelength.
The absorption spectrum can be used to identify a compound since it is specific to the molecule
absorbing the radiation (molecular fingerprint).
Quantitative determinations are based on the direct relationship between the absorption of
electromagnetic radiation and the concentration of the absorbing species in the solution
C.
Absorption of Light and Quantitative Determinations
The absorption or transmittance of electromagnetic radiation within a narrow range of the
electromagnetic spectrum can be measured using specifically designed spectrophotometers. In
this experiment, the light absorbed by molecules, which absorb in the visible region of the
electromagnetic spectrum, will be studied using a spectrophotometer that detects changes in
absorption in this region. Solutions that are colored absorb light within the visible region of the
electromagnetic spectra (i.e. 380 –780 nm). The color of the solution is determined by the
wavelength of light that is transmitted by the solution (i.e. not absorbed).
