20.1 Huygens' Principle

christian Huygens, a Dutch Physicist and contemporary of Sir Isaac Newton, formulated a principle or way of explaining the propagation of light waves that is still very useful today. Huygens' Principle is that the future location of a wave is determined by considering each point on the present wave as a source for new wavelets. The new, future wave is the envelope of these wavelets. The envelope of these wavelets is a line drawn across them to contain them and it accurately locates the future wave. This idea is best understood by the illustrations in Figure 20.1. There a plane wave moves to the right and a circular wave expands from its origin.

Figure 20.1 Each point on a wave may be treated as a source for new "wavelets". The new position of the wave is the envelope of these little wavelets. This is Huygen's Principle.

In Figure 20.1 the illustrations are symmetric and Huygens' Principle simply confirms what we would expect. But Figure 20.2 shows a plane wave passing through a hole in a barrier. There a barrier blocks parts of the wave and the symmetry is broken. Some of the wavelets are stopped. The envelope of the remaining wavelets spreads around the edges of the hole; the wave bends past the edges of the hole. This behavior is known as diffraction and is characteristic of waves-of all waves. As the hole becomes smaller this bending or diffraction becomes greater.

Figure 20.2 Plane waves encounter a barrier with an opening. All of the wavelets necessary to create a new plane wave are not present. The resulting wave bends into the region that appears to be blocked.