Lecture 12 summary

Basic concepts of spectrophotometry (in progress)

The electromagnetic (EM) spectrum and the properties of light

The EM spectrum encompasses a wide range of types of radiation that are familiar to us in a variety of different contexts - radio waves, visible light, X-rays - but these are all fundamentally the same physical phenomenon. Thus visible light represents only a small portion of the entire EM spectrum.

Relationship between wavelength (l) and frequency (n) of electromagnetic radiation:

c  =  ln.

where c is the speed of light (c = 2.998 x 10⁸ m/s).

The relation between energy and frequency (Planck's relation): E = hn ; The quantity h is Planck's constant, a fundamental constant of nature that corresponds to the quantization of matter and energy at the atomic scale. The value of Planck's constant: h = 6.627 x 10^–34 J s.

We can derive a relation between the energy and the wavelength of EM radiation by combining the above two relations. Substituting n = c/l into Planck's relation yields E = hc/l . In spectrophotometry, the quantity wavenumber is somettimes used (symbolized n-bar) which is defined as 1 /l . Thus, while wavelength is inversely-proportional to energy, both wavenumber and frequency are directly proportional to energy.