Huygens's Principle


The Dutch scientist Christiaan Huygens (1629 – 1695) is famous for three great accomplishments: he discovered the true form of the rings of Saturn; he invented the first practical pendulum clock; and he was the first person to try to explain the propagation of light by using a wave model. He did important work in mechanics, and much practical optics; the type of eyepiece commonly used in microscopes was invented by him.

Of course, the Web is full of pages about him; here are a few:

A standard place to look for biographies of scientists is the collection at Rice University, and of course they have all the basic information on Huygens neatly laid out.

The science museum in Florence has a good one-paragraph biography of Huygens.

The history-of-science site at St. Andrews has a fine and rather extended account of Huygens's work and life, but it may be too extensive for the casual reader.

F. J. Dijksterhuis, an historian of science specializing in Huygens at the University of Twente, has a splendid series of pages devoted to Huygens (including a sound file with the correct pronunciation of his name). Highly recommended!

It would be nice if everyone would spell the name Huygens correctly. Many people feel obliged to add an extra “h” to it.

The Principle

Huygens's Principle is a simple method of constructing the position of a wave at successive times. As it is well explained elsewhere, I'll simply offer a few links here:

The American Heritage Dictionary has a clear statement of the principle on (They also have a brief entry on Huygens himself.)

A pretty standard “textbook” illustration is shown here.

Walter Fendt has a nice Java applet that illustrates how the principle works. You have to press the green “next step” bar before anything interesting happens. The details get more complex each time you click this bar. Eventually it hung my browser; but I was still glad I follwed the demo through to the end. Another copy, where the “next step” bar is yellow, is in Germany.

A more mathematical treatment is available at, in the “Physics” section. If you're at ease with partial differential equations, you should be delighted with this treatment. If you have no idea what a differential equation is, a glance at the first screen will be enough for you.


© 2002 – 2005, 2006, 2012 Andrew T. Young

Go back to the ...
main optics page
or the astronomical refraction page