Spectroscopy uses various forms of electromagnetic radiation for analysis. Electromagnetic radiation consists of massless particles (photons) traveling in a wave-like pattern and moving at the speed of light.  Major difference between types of electromagnetic radiation is the energy of the photons--radio waves possess lowest while gamma-rays highest.

Spectroscopy comprises absorption, emission, or scattering. The transition of an electron from lower to higher level is called absorption; transition from higher to lower level is called emission; redirection of light due to interaction with matter is called scattering (usually results in slightly different wavelength l).
 
 

Spectrum Discovery
Isaac Newton (1666) passed a beam of sunlight through a prism and produced a band of colors just like the rainbow. He then passed each of these colors through other prisms and found they did not change.  When Newton passed the whole band of colored lights through a prism in reverse order, the colored band became white sunlight again. Newton concluded that white light is really a mixture of colored lights and each color bends by a different amount when passing through the prism. The band of colored lights discovered by Newton is called a spectrum; the rainbow is actually a spectrum formed by sunlight passing through raindrops. Ironically, Newton believed the spectrum to be his most important discovery.
 

Dispersion is the separation of light into its colors by refraction (bending) of the light through a prism. Each of the colors has its own wavelength which determines how much each color will bend; red bends least and violet most.
 
 
 

Utilizing the dispersive action of the prism, Gustav Kirchhoff and Robert Bunsen built the first spectroscope (1859) and studied the patterns produced by elements vaporized in a Bunsen flame. Elements such as hydrogen, lithium, and mercury exhibit the following bright-line spectra:

Using this method of spectrum analysis led to the discovery of cesium/rubidium/thallium. Spectra were soon obtained for the different elements and their lines charted.

English astronomer Norman Lockyer (1868) noted lines in the solar spectrum which could not be identified with charted lines of any known element. Lockyer believed an unknown element existed in the sun and named it helium from the Greek word for sun.  From its spectrum, William Ramsay (1895) identified the presence of helium on earth.

Spectral lines were believed to originate from the vibration of atoms at different rates under the stimulus of heat.  The faster vibrations resulted in shorter waves causing lines to appear toward the violet end of the spectrum while slower vibrations gave lines toward the red end.  In 1885 Johann Balmer discovered through experimentation that the various rates of vibration in a mass of glowing hydrogen gave a simple mathematical relation to each other. This indicated that some one type of "mechanism " was at work at varying rates within the hydrogen atom, giving off the different wavelengths. Johannes Rydberg developed the following equation to describe many observed lines:
          911A/l  = 1/n² - 1/m²

In 1913 Niels Bohr proposed the hydrogen atom consisted of an electron revolving around a central nucleus.  Using Planck's constant and the Rydberg equation, Bohr explained the spectrum of hydrogen. As an atom absorbed energy, this orbit would enlarge by definite amounts. When energy was emitted, as in the form of light, the electron would fall by steps into inner orbits, and the frequency of the light would depend upon how many orbits were traversed.

Modern spectroscopes vary considerably in function and design and are often quite specialized for the specific substances they analyze. Teaching instruments used in today's classrooms are the relatively simple prism spectroscopes shown to the left. They consist of a collimator (tube for admitting light), a glass prism, and a telescope. The collimator has a slit at one end to admit light and a lens on the other to concentrate it. The lens directs the light on the prism, which disperses the ray into its component colors.
 
 
 
 

Applications 
Analytical techniques based on infrared, ultraviolet, and visible radiation are used in modern labs.  The SPEC 20 can use visible light to determine the concentration of CuSO4 solutions.  Since CuSO4 transmits in the blue region (4000-4500A), it absorbs in the red region (6500-7000A).  If the concentration of sample #1(C1 = 1.00M) is known and its absorption measured (A1 = 0.600), the concentration of sample #2 (C2) can be determined from its absorption (A2 = 0.300) and Beer's Law. Beer's Law describes how the intensity of light diminishes as it passes through an absorbing media.
C₂A₁ = C₁A₂
C₂ = (1.00M)(0.300)/(0.600) = 0.500M
Infrared spectroscopy generates spectra by graphing absorbance versus wavelength.  The spectra for four salts are provided below.  Computers are able to quickly identify compounds by comparing the obtained spectrum with known spectra.