2. BASIC PROPERTIES OF ELECTROMAGNETIC RADIATION

All types of electromagnetic radiation share common properties:

Speed of Propagation: in empty space, all types of electromagnetic radiation travel at a speed of c = 300,000 kilometers/second (to be exact, it's 299,792,458 meters/second). That's fast! For example, in a telephone call from Denver to London, there is a delay of about 0.04 seconds, the time it takes for light to travel through a fiber optic cable of length about 10,000 km. (Actually, light slows down by about 30% when it travels through glass.) Below is a table of distances and times for light (or any other kind of electromagnetic radiation) to travel various distances:

Because astronomical distances are so great, we often use the light-travel time to characterize the distance. Thus we can say that the Moon is at a distance of 1.3 light-seconds from Earth, and the nearest star, 4.3 light-years. (Think about the problems of operating a robot on Jupiter from Earth, compared to one on the Moon.) Whenever you see "light-(time)", that means we are talking about distance, not time. Nowadays, astronomers routinely observe galaxies at enormous distances, ranging up to 10 billion light-years or more!

Wavelength and Frequency: all kinds of electromagnetic radiation are characterized by either wavelength, l, or frequency, f. The higher the frequency, the shorter the wavelength, as illustrated by this Java Applet from the University of Colorado Physics Department. They are related by the formula lf = c, where c stands for the velocity of light; therefore, once you specify the frequency, the wavelength is set, and conversely.

Interaction with matter: we call it electromagnetic radiation because it (any type) is emitted and absorbed by electrically charged matter or electric currents (which always produce magnetic fields). For example, radio waves are emitted and absorbed by currents of electrons that flow through radio antennae, and optical radiation is emitted and absorbed by atomic electrons.

Wave-particle duality: all types of electromagnetic radiation act as both waves and particles. We make use of the wave-like properties such as interference and refraction in designing instruments (spectrometers) to separate radiation containing many different wavelengths into a spectrum (see below). But radiation also acts as particles that we call photons. Photons are the minimal chunks of radiation of a given frequency. You can't keep dimming down light of a given color (say, red) indefinitely and see a steady red signal. Eventually you will see the red photons arriving one-by-one, in distinct little red flashes at irregular intervals. The particle nature of light is most evident in detectors (see Lesson 2) that convert photons into electrical signals.

This wave-particle duality of electromagnetic radiation raises deep philosophical problems that challenge our intuition about the nature of cause and effect. It turns out that particles of matter, such as electrons, also behave as both wave and particle. To appreciate these puzzles and understand their solution, and how light and atoms interact, you must learn quantum mechanics. (A photon is a quantum -- or irreducible quantity -- of electromagnetic radiation.) Quantum mechanics is a fantastic subject, but beyond the scope of this course.

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Last modified October 18, 2002;
Copyright by Richard McCray