Electromagnetic Waves & Electromagnetic Spectrum - Benjamin Crowell - GFDL license
What Are Electric Fields and Magnetic Fields?
To understand electromagnetic waves, one must first understand electric and magnetic fields and how they interact. Electric fields make electric charges attract or repel each other. Similarly magnetic fields cause magnetic forces.There are two basic types of electric charges. They are positive and negative. In atoms, protons have positive charges and electrons have negative charges. Electric forces between the protons and electrons cause the electrons to orbit the nucleus. Like charges, such as two protons or two electrons, repel each other. Unlike charges, such as a proton and an electron, attract each other. These attractive or repulsive electric forces are governed by Coulomb's Law.
The protons and electrons are not actually in contact, so how can they exert forces on each other? We can't push or pull something without actually touching it, so why should electric charges be able to? The answer is electric fields. Any electric charge creates an electric field. An electric field exerts a force on another electric charge. Electric charges exert forces on each other via electric fields.
How Electric and Magnetic Fields Interact
Wrap a wire around a nail and connect the wire to a battery. When an electric current passes through the wire, the nail becomes magnetic. Why? The electrons in the wire create electric fields. When there is an electric current the electrons are moving through the wire. Moving electrons set up changing electric fields. Changing electric fields cause magnetic fields. The nail becomes magnetic. All magnetic fields are in some way caused by changing electric fields. In a similar way changing magnetic fields, like electric charges, cause electric fields.Note the symmetry. Changing electric fields cause magnetic fields. Changing magnetic fields cause electric fields.
Electromagnetic Waves
If electrons are moving in a wire, say a radio transmitting antenna, they will set up changing electric fields. The changing electric fields set up magnetic fields. These magnetic fields are also changing, so they set up changing electric fields. In this way the electric and magnetic fields oscillate and propagate through space. They form an electromagnetic wave, also called electromagnetic radiation.In an electromagnetic wave the electric and magnetic fields are mutually perpendicular. They are also both perpendicular to the direction in which the wave propagates or travels. The electric and magnetic fields oscillate together between maximum positive and maximum negative values. The frequency of these oscillations and the wavelength of the waves determines whether the electromagnetic wave is visible light (and its color), ultraviolet light, infrared light, radio waves, X-rays, or gamma rays. All are forms of electromagnetic waves.
Maxwell's Equations
In the mid 19th century Scottish physicist, James Clerk Maxwell, noticed the symmetry and other properties of electric and magnetic fields. He brilliantly summarized electromagnetism in four equations, now known as Maxwell's equations. Mathematically manipulating these equations, Maxwell found the equation that describes waves. He had theoretically predicted electromagnetic waves. Noticing that the speed of these theoretically predicted waves was the same as the speed of light, Maxwell deduced that light was an electromagnetic wave and predicted the existence of other electromagnetic waves. These other electromagnetic waves including ultraviolet and infrared light, X-rays, microwaves, and other radio waves comprise the electromagnetic spectrum.Further Reading
Knight, R.D., Physics for Scientists and Engineers, Pearson, 2004.Strobel, N., AstronomyNotes - has a nice animation of an electromagnetic wave.
Copyright Paul A. Heckert. Contact the author to obtain permission for republication.
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