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Electromagnetic Radiation


Do you listen to the radio, watch TV, or use a microwave oven? All these devices make use of electromagnetic waves. Radio waves, microwaves, visible light, and x rays are all examples of electromagnetic waves that differ from each other in wavelength.


(a) Longer wavelength; (b) shorter wavelength.


Electromagnetic waves are produced by the motion of electrically charged particles. These waves are also called "electromagnetic radiation" because they radiate from the electrically charged particles. They travel through empty space as well as through air and other substances.


Scientists have observed that electromagnetic radiation has a dual "personality." Besides acting like waves, it acts like a stream of particles (called "photons") that have no mass. The photons with the highest energy correspond to the shortest wavelengths.


The electromagnetic spectrum covers a wide range of wavelengths and photon energies. Light used to "see" an object must have a wavelength about the same size as or smaller than the object. The ALS generates light in the far ultraviolet and soft x-ray regions, which span the wavelengths suited to studying molecules and atoms.


Electromagnetic radiation consists of discrete packets of energy, which we call photons. A photon consists of an oscillating electric field component, E, and an oscillating magnetic field component, M. The electric and magnetic fields are orthogonal (perpendicular) to each other, and they are orthogonal to the direction of propogation of the photon. The electric and magnetic fields of a photon flip direction as the photon travels. We call the number of flips, or oscillations, that occur in one second the frequency, . Frequency has the units of oscillations per second, or simply s-1 (this unit is given the name Hertz). If the electric and magnetic fields of a photon could be recorded as the photon traveled some distance, it would leave the trail of E and M fields shown in the figure. All photons (in a given, non-absorbing medium) travel at the same velocity, v. The physical distance in the direction of propogation over which the electric and magnetic fields of a photon make one complete oscillation is called the wavelength, , of the electromagnetic radiation. The figure above shows the hypothetical history of a photon that has traveled the distance of one wavelength.




The nature of matter