The Electromagnetic Spectrum

Light is composed of electric and magnetic fields and thus its alternate name "electromagnetic spectrum". Light can be absorbed and emitted by atoms and molecules. The precise details of which give us the world we see around us. Chemists are interested in the details of absorption and emission of light by matter because these details can be viewed as fingerprints of atoms and molecules as well as their chemical properties. To understand the interaction of light with matter we need to first understand the basic character of light

Light is a wave

Traditional physics views light as a wave - with all the properties usually associated with waves. For example light can undergo constructive and destructive interference - just like ripples on a lake.

Light is characterized by its wavelength and frequency

Since light has the characteristics of a wave then we can associate a wavelength (l) and frequency (n) to it: The wavelength is the physical distence between two successive crests of the wave and the frequency is the number of times the wave oscillates per second (Hz).

The frequency and wavelength of light are related to one another by the speed of light c

According to Einstein's theory of special relativity nothing travels faster than light. The speed of light (c) in a vacuum (e.g., outer space) is 2.998x10+8m/s. The speed of light is related to the wavelength and frequency by the equation

l n = c

Since the speed of light does not change, if we know the wavelength then through this equation we also can determine the frequency and visa versa.

Light has energy

The energy of light is proportional to its frequency and inversely proportional to its wavelength. The proportionality constant is known as Planck's constant h.

Energy = hn = h c/l

h=6.627x10-34 Joule second

The Spectrum

Light comes in a huge variety of wavelegths. The visible part of this spectrum (the part that can be seen with the human eye) is only a very small part of this spectrum. This spectrum, along with the sources of the radiation, are given below.

 l (m)-> 10-12 10-10 10-8 10-7 10-6 10-5 10-2 1 gamma radiation X-rays far UV UV Visible IR micro- wave radio nucelar events innner core electron high E outer elelctron outer electron low E outer electrons molecular vibrations molecular rotations transmitters

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C101 Class Notes
Prof. N. De Leon